If I add 20% to a number and end with 90, what number did I start with?
N + .20N = 90 simplify
1.20N = 90 divide both sides by 1.20
N = 75 = the number you started with
Verify
75 + 20% of 75 =
75 + (20/100)*75 =
75 + (1/5)*75 =
75 + 15 =
90 ....
120%=90
1%=90/120
100%=90/120*100=75! Just like what CPhill got!
+168 
