# If a family spends $408 of their monthly income on food which is 20% of their income, what was their monthly income?I knwo with percentage it's: amount over base is to percent over 100.So, given...

If a family spends $408 of their monthly income on food which is 20% of their income, what was their monthly income?

I knwo with percentage it's: amount over base is to percent over 100.

So, given that would it read: $408 is to b as 20% is to 100 (or $408 over b=20% over 100) And then you cross multiply, yes? If not please tell me what I'm doing wrong. :)

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### 3 Answers

If we were finding a percentage, we would have part divided by whole multiplied by 1000 which would look something like this

percent = 100 * part / whole

In this problem, we know the percentage, and we know the part. what we don't know is the whole. We can substitute in the values that we do know (part and percent) to determine the value of the whole (i.e. the monthly income). For now, we'll substitute x in for the income.

20 = 100 * 408 / x

Multiply both sides by x

20x = 40800

then divide both sides by 20 to find the value of x which will be their monthly income.

x = 2040

408/x = 20/100

cross multiply:

20x = 40800

divide by 20

20x/20 = 40800/20

x = 2040

$408 of their monthly income on food which is 20%

408/x = 20/100

cross multiply:

20x = 40800

divide by 20

20x/20 = 40800/20

x = 2040

their monthly income 2040