In mathematics, the word 'of' always means multiplication, so to find out what 15% of $10.00 is, all we need to do is multiply the percentage by the dollar amount we are reducing.
In order to do this, we must first convert the percentage into a decimal. To do this, we can either divide the percentage by 100, or (my personal shortcut) just shift the decimal point over two places to the left.
15% = 0.15
So now, we can multiply.
0.15 * 10 = 1.5
which is our answer.
15% of $10.00 is $1.50.
Hope that helped! =)