B How do you multiply percents on paper

[jim1174]

5 million times 5 percent would be 5000000 X 0.05. When you do this on a calculator you get 250000 but when you do it on paper you end up with more zeros in the answer. When you multiply percents on paper what is the easiest way to figure out how many zeros go in your answer?

 

[jfizzix]

When it comes to computing math by hand, I like fractions more than decimals.

5 percent is 5/100, so I'd multiply 5 million by 5 to get 25 million, and divide by 100 to get 250 thousand.

If you're getting more zeroes, you may be getting 25000000, but because it's 0.05 instead of 5, you need to shift the decimal point in your result two places to the left.

 

[fresh_42]

5 million times 5 percent would be 5000000 X 0.05. When you do this on a calculator you get 250000 but when you do it on paper you end up with more zeros in the answer. When you multiply percents on paper what is the easiest way to figure out how many zeros go in your answer?

First and most important is that % means nothing else than divided by 100.
Next it is best to write such numbers by powers of ten: $5000000=5,000,000 = 5\cdot 10^6$. Thus we have
$$
5000000 \cdot 0.05 = 5 \cdot 10^6 \cdot 5 \cdot \frac{1}{100} = 25 \cdot 10^6 \cdot 10^{-2} =25 \cdot10^4 = 250,000
$$

 

[FactChecker]

Factor out the powers of 10 (move the decimal point) so that you can add or subtract exponents: $5000000 * 0.05 = 5*10^6*5*10^{-2} = 25*10^{6-2} = 25*10^4 = 250000.$

EDIT: I'm too slow. ;>)

 

[coolul007]

It is a rule of decimal long hand multiplication. Count all the places to the right of the decimal point in both numbers, in your case 2 places(0.05) and 0 int the original number. the answer has to a decimal point 2 places to the left.
example: 0.125 x 1.34 = 125 x 134 = 16750 now we need to add 5 decimal places: 0.16750 or 0.15675