92 Part 1: The World of Numbers
Calculating with Percentages ..............................................................................................
Because percentages are really ratios with a denominator of 100, they can easily be changed to
fractions or decimals. To change a percentage like 44% to a fraction, just remember that percent
means “out of 100.” 44% =
44
100 , which simplifies to11
25. To change a percentage to a decimal, drop
the percent sign and move the decimal point two places to the left. 36.8% = 0.368 and 3% = .03.
If you have a fraction and want to change it to a percentage, you can use a proportion as you did
earlier, or, if you can conveniently change it to a denominator of 100, you can let it tell you what
percent it is. The fraction
1
425
100
so it equals 25%, but 136 doesn’t easily convert to a denominator
of 100, and it would be better to use the proportion method.
To change a decimal to a percentage, move the decimal point two places to the right and add a
percent sign. 0.15 = 15% and 8.93 = 893%.
Yes, you can have a percentage greater than 100 percent. 100 percent is the whole thing.
Percentages greater than that say you have more than the whole thing. If this week you earn
150 percent of what you earned last week, you earn all of what you earned last week (100%) and
another half of that (50%).CHECK POINT- Change 42% to a fraction.
- Change 85.3% to a decimal.
- Change^52 to a percentage.
19. Change 0.049 to a percentage.
20. Change 5.002 to a percentage.
Calculating Interest
Much of the time, when you encounter percentages, it’s in some kind of financial calculation.
It might be a big deal, like the interest you’ll pay on your mortgage, or the interest the bank will
pay you on your savings. Or it might just be leaving an appropriate tip for your server when you
eat out.
Interest is money paid for the use of money. If you borrow money, you must pay it back plus
additional money as a fee for the loan. If you deposit money in a bank account or invest money,
you receive interest for allowing the institution to use your money. This cost of the loan is
usually calculated as a percent of the amount you borrowed.