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### Course: Pre-algebra > Unit 11

Lesson 8: Scientific notation word problems# Scientific notation word problem: U.S. national debt

Ever wonder what your part of the national debt is? It might surprise you. What isn't surprising is that you can use scientific notation and division to figure out the answer. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- I often wonder about when is the correct time to round a number during a calculation. At3:50you rounded to the fourth decimal place. Should you wait to round until you have moved the decimal for the last time? Making it 3.9786*10 to the 4th?

I have gotten the answer wrong by not rounding during the problem, but it is less accurate too.

Am I correct in rounding after, or should it occur during the problem calculation?(17 votes)- You should estimate (round) an answer only at the very end, not before that.

Although, for most of the calculations, rounding up to the 6th or 7th place is enough to still get a precise answer at the end. I'd just like to make it clear that this is NOT a fact, this is only my personal opinion based on thousands of exercises I've done.(22 votes)

- Little did they know that in 13 crazy years later the national debt would 3x.....(25 votes)
- In2:17he changes 10^8 into 10^-8. How does this work. Wont that change the outcome? Please explain.(12 votes)
- So, look carefully between2:05and2:20- in first case you have a fraction (10^13)/(10^8), where 10^8 is under the fraction; afterwards, Sal transformed the division into multiplication, where the number under the fraction is with a negative exponent, tha's how 10^13 divided by 10^8 becomes 10^13 multiplied by 10^-8;(5 votes)

- I don't understand, where did you get the number 9 when the number said .3978 (in the calculator)(6 votes)
- When he divided 1.2278 by 3.086, The calculator gave him: .397861 The question said to round to four decimal places. The number in the fourth place is 8, so when you want to round it, look at the number to the right of it ( which is the lesser number) That number is 6. If the number is 4 or below, keep the number the same, if it's 5 or above, round the number up. Since 6 is above 5, you would round the 8 to a 9.(13 votes)

- At5:10, does everyone really owe that much or do some people owe more than others?(3 votes)
- Our national debt is not directly allocated to individual people. You will sometimes seem problems like the one in the video that show the average amount every person would owe to pay of the debt. This gives you a sense of how large the debt is.

In reality, our federal income tax is used to pay for federal expenses. Individuals and companies pay income tax. And, the rates are based on income levels. So, the average cost per person does not directly align to how taxes are paid.(6 votes)

- I appreciate the way Sal differentiates terms with colours, really helpful!

...though I admit it's rather jarring when the realistic calculator spawns onscreen(5 votes) - why did the eight turn into a negative 8 in2:30?(3 votes)
- Because 1/10⁸ is the same as 10⁻⁸.

When you move a number with an exponent from the numerator to the denominator (or from the denominator to the numerator), you must change the sign of the exponent.(4 votes)

- In 2022, the gross federal debt in the United States amounted to around 92,528 U.S. dollars per person (statista.com). 12 years and we tripled our debt!(4 votes)
- Around2:36Sal said 10^13/10^8 = 10^5 , What if it was 10^-13/10^8 or 10^13/10^-8.. Do I subtract regularly like 13-(-8) or -13-8?(4 votes)
- 10^-13/10^8 = 10^-13 * 10^-8 = 10^-21

10^13/10^-8 = 10^13 * 10^8 = 10^21(3 votes)

- At3:18did Sal say it wrong. 1.2278 divided by 3.086 is 0.3978 (with four decimals) Not 0.3979. So the wouldn't the correct answer is 3.978 multiplied by 10 to the power of 4.(2 votes)
- no he did it right, he was rounding, thats why he got 0.3979 instead of 0.3978.(5 votes)

## Video transcript

On February 2, 2010 the U.S.
Treasury estimated the national debt at 1.2278 times
10 to the 13th power. And just to get a sense of
things, 1 times 10 to the sixth is a million, 1 times 10
to the ninth is a billion, 1 times 10 to the 12th
is a trillion. So we're talking on the
order of magnitude of 10 trillion dollars. So this is about 12
trillion dollars. Then they tell us that the U.S.
Census Bureau's estimate for the U.S. population was
about 3.086 times 10 to the eighth power. So this is a little over
300 million people. So that's an interesting number
right there, it's the population. And then they say, using these
estimates calculate the per-person share of
the national debt. So essentially, we want to
take the entire debt and divide by the number
of people. That'll give us the per-person
share of the national debt. Use scientific notation to make
your calculations and express your answer in both scientific and decimal notation. Which means just as
a regular number. Round to four decimal places
while making calculations. So we want the per-person
debt. So we want to take the total
debt and divide by the number of people. So the total debt is 1.2278
times 10 to the 13th. And we want to divide that by
the total number of people, which is 3.086 times
10 to the eighth. And we could separate this into
two division problems. We could say that this is equal
to the division right here, 1.2278 divided by 3.086. And then times 10 to the 13th
divided by 10 to the eighth. Now, what's 10 to the 13th
divided by 10 to the eighth? Let me do it over here. The way I think about it, this
is the exact same thing as 10 to the 13th times 10 to
the negative eight. This is an eight right here. If you have a 10 to the eighth
in the denominator, that's like multiplying by 10 to
the negative eight. So you have 13, you the same
base 10, so 10 to the 13th times 10 to the negative eight
is going to be 10 to the 13 minus 8. Which is 10 to the fifth. Or another way to think about
it: If you have the base in the denominator, you subtract
the exponents. So it's 13 minus 8. 10 to the fifth. So it's this blue expression
times 10 to the fifth. And let's get a calculator out
to calculate this right here. And they say round everything
to four decimal places, so I'll keep that in mind. Let me turn my calculator on. 1.2278 divided by 3.086
is equal to 0.3979. Because we want to round
right there. Let me remember that. Or let me just put it
on the side so I can still look at it. So this this little
dividing decimals problem results in 0.3979. And of course, times 10 to the
fifth dollars per person. Once again, you might be tempted
to say, hey this is in scientific notation. I have some number times
a power of ten. But notice, this number is not
greater than or equal to 1. Remember, this number, if you
want to be formal about scientific notation, has to be
greater than or equal to 1, or less than 10. So what we can do here
is we can multiply. If we don't want to change the
number, we can multiply this number by 10 and divide
this number by 10. Or another way you can think
about it is, this whole thing can be rewritten as
0.3979 times 10 times 10 to the fourth. What I did was just now was I
broke up the 10 of the fifth into a 10 and a 10
to the fourth. And I did that because I want
to multiply this by 10 so I can get a 3 out front
instead of a 0.3. So let's do that. So essentially, I took a 10 out
of the 10 to the fifth. I divided it by 10, I multiplied
this other guy by 10, not changing the
whole number. So then this right here will
become 3.979 and then times 10 to the fourth. So that's how much debt
there is per-person in scientific notation. So this is debt per person in scientific notation. Now, in the problem they also
wanted us to express it in decimal notation. Which is just kind of standard
writing it as a number with our standard numeric
decimal system. So what is 3.979 times
10 to the fourth? Let's think about it. We have 3.979 times ten to the fourth. Well let me just
do it this way. Let's just move the
decimal space. If we multiply it by 10, we're
going to get 39.79. If we multiply it by
10 squared, we're going to get 397.9. If we multiply it by 10
to the third, we're going to get 3,979. If we multiply it by 10 to the
fourth, we're going to get one more zero right there. So we're essentially
going to move the decimal four to the right. So I could write it like this. This is equal to $39,790. So if you think about the
national debt per person. Every man, woman, and child in
the United States essentially owes $39,790.