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### Course: Algebra basics>Unit 6

Lesson 5: 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. At you 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?
• 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.
• Little did they know that in 13 crazy years later the national debt would 3x.....
• In he changes 10^8 into 10^-8. How does this work. Wont that change the outcome? Please explain.
• So, look carefully between and - 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;
• I don't understand, where did you get the number 9 when the number said .3978 (in the calculator)
• 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.
• At , does everyone really owe that much or do some people owe more than others?
• 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.
• 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!
• I appreciate the way Sal differentiates terms with colours, really helpful!
...though I admit it's rather jarring when the realistic calculator spawns onscreen
• Around Sal 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?
• 10^-13/10^8 = 10^-13 * 10^-8 = 10^-21
10^13/10^-8 = 10^13 * 10^8 = 10^21
• why did the eight turn into a negative 8 in ?