Question

12 is 30% of ____
(Please explain)

Answer 1

40

Explanation:

In word problems, the word "of" usually means multiply. Thus, for example, 10% of $50 is $5. This is because we can write 10% as 0.10 or as 10/100 (which is 1/10) and then multiply: (1/10)(50) = 5

So, we often see equations like:

R = (1/2)D meaning R is 50% of D, or R is half of D

Let x = the number we want to find

This problem says:

"12 is 30% of what number?"

12 = (30/100) * x

12 = (3/10)x

(10/3)12 = x [multiply both sides by (10/3)]

40 = x

Checking (very important):

Is 12 = (3/10)(40) ?

12 = (3)(4) ?

12 = 12 ? yes

Answer 2

40

Explanation:

Let x = unknown number

30% = 30/100 = 0.3

If 12 is 30% of x, then

30% of x is 12

⇒ 0.3x = 12

⇒ x = 12 ÷ 0.3

⇒ x = 40

Or, you can divide 12 by 3 to get 10% and then multiply this by 10 to get 100%:

12 ÷ 3 = 4 (so 4 is 10%)

4 × 10 = 40 (which is 100%)