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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
Hope this is helpful.
• 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
• It's asking how many individual red blood cells does it take to make up the total volume of blood cells (2 liters), if each blood cell is 90x10^-15 liters. Multiplying 90x10^-15 by 2 would give you the volume of 2 red blood cells.
• 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!
• Is anyone else confused
• Probably
• At , Sal wrote "5 (liters) x 40%". Can someone show me how to work this out? ( You don't have to include the liters)
• At , why can 2/9*10^-14 be rewritten as 2/9 * 1/10^-14?

I can calculate out that it's equivalent (2*1 in the numerator positions equals 2, as per the original version). But in the initial version, aren't both the 9 and the 10^-14 being divided into 2? Why wouldn't the numerator be 2 in both of these fractions? (Excepting the fact that 2*2 = 4 not 2. I'm trying to understand what's actually happening here as opposed to getting a recitation of a rule.) Thanks!