Working with Percentages and Proportions
When working with percentages, it is often helpful to think of them in terms of ratios
and proportions. For instance, if someone asks you, â€œWhat is 10% of 40?â€ you could
simply change 10% into a decimal (0.1) and multiply it by 40 to get 4. But what if you
were asked, â€œ5% of what number is 10?â€ Thatâ€™s a little trickier. To do this calculation, it
is convenient to use a proportion.
PROCEDURE: To use percentages in a proportion, first put your known percentage in
a ratio with 100. Then create an equivalent ratio, leaving the place for your
unknown quantity blank. Cross-multiply the known numerator with the known
denominator. Divide the product with your remaining known value. The result is
your unknown quantity.
SAMPLE PROBLEM: 25% of what number is 4?
|Step 1: Put your percentage in a
ratio with 100.
|Step 2: Create an equivalent ratio, leaving
the space for the unknown quantity blank.
|Step 3: Cross-multiply the known
numerator with the known
|Step 4: Divide the product with the remaining
25% of 16 is 4.