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### Course: Grade 8 math (FL B.E.S.T.)>Unit 1

Lesson 9: Scientific notation word problems

# 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
• 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.
• why would a math student want to know how many red blood cells are in the human body??!! I mean i get it for the vampires but.........
• Because it is just a scientific number that we are learning
• Is anyone else confused
• Probably
• 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?
(1 vote)
• 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!
• How did 10^-14 became 10^14?
• The 10^(-14) is in the denominator, so it is 1/10^(-14). Properties of exponents tell us that 1/x^(-n) = x^n/1 of just x^n. We can change from a negative exponent to a positive exponent by using the reciprocal of the base.
Thus, 1/10^(-14) = 10^14/1 or just 10^14. When this is multiplied with the other parts of the fraction, the 10^14 ends up in the numerator of the original fraction.

Search for "negative exponents" to get more details about working with them.
Hope this helps.
• At , Sal wrote "5 (liters) x 40%". Can someone show me how to work this out? ( You don't have to include the liters)