Mary was making $25,000 per year. her boss gave her a $3,000 raise. what percent increase is that? plz

The easiest way to find a percentage is to divide the two numbers. You're trying to find what percent 3,000 is of 25,000, so you divide 3,000 by 25,000 to find the decimal.
3000/25,000 = 0.12
If you move the decimal point to the right two places, you'll have your percentage. (Percent means "out of one hundred", and since the decimal point is now in what was the hundredths place, it is now a percentage instead of a decimal.)
12%
$3,000 is a 12% raise from $25,000.