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# Scientific notation word problem: red blood cells

Vampires and math students want to know: How many red blood cells are in the a human body? We can find the answer using scientific notation. Created by Sal Khan.

## Want to join the conversation?

• Why wouldn't you just divide the 9 into 2 and have the quotient of 0.22 there instead of ''borrowing'' the 10?
• You could do that, and have 0.222 * 10^14. But then, to get it into official scientific notation, you would need change it to 2.22 * 10^13. So Sal is just doing the conversion into scientific notation in the earlier step. In both cases you would end up with the same answer,
• Why was 1/10^-14 the same as 10^14? Not quite sure.
• Well, you can think it as (1/10)^-14. Then, according to the rule (a/b)^c = ((a^c)/(b^c)), you can use the distributive property:

((1^-14)/(10^-14)
When a fraction has either a numerator or denominator or both with a negative exponent, you need to switch the position. Meaning the numerator with a negative exponent would switch to the denominator for a positive exponent, and vice versa.
((1^-14)/(10^-14)) = ((10^14)/(1^14))
And since we know that 1 with a positive exponent is still one, it becomes:
(10^14)/1
And since division by one means the quotient is the dividend, it becomes:
10^14
• At in the video, you start to mention that 90 isn't in scientific notation because it isn't less than ten, which I agree with, but under that understanding, wouldn't 10^-15 technically not be in sci.n. because it isn't less than ten?
• 90 isn't in scientific notation because the coefficient isn't less than 10. Since 90 is just a number, 90 is the coefficient which isn't less than 10.

With 10^-15, the coefficient is 1 which is less than 10.

However, it should be noted that to be in scientific notation, the coefficient must also be greater than or equal to 1.

I hope this clarifies what Sal meant!
• I realize I am the only person having trouble understanding this, but why did we divide by volume of 1 red blood cell rather than multiplying? total volume of 1 red cell/ volume of 1 cell = # of blood cells??

I understand the calculation itself just confused on the formula
• At , Sal wrote "5 (liters) x 40%". Can someone show me how to work this out? ( You don't have to include the liters)
• this is old
also how did he get the ten to turn into a nine that is some weird math right there. Does he have anymore things involving this kind of math because i still need to understand more.
• At in the video, couldn't Sal have done 2/9 first, then made it into scientific notation, rather then multiplying it by ten? I'm kind of confused, whether I'm doing it right or wrong. Does both ways always work?
• Is anyone else confused
• Probably