ok so to solve for percent off
percent off means discount
so therefor we need to find how much was discounted
30-20.10=9.90
so we see that 9.90 was discounted
to find the percent we do
discount/total=percent discount
9.90/30=0.33
percent means parts out of 100
0.33/1 times 100/100=33/100=33%
answer is 33% discout
