I really had to do mental math this would be my thought process
$100, eight percent of if would be 8 dollars
Then I'll figure out how much is ten percent of $10 which is $0.8 and add 0.8 + 0.8 together to get 8 percent of $20
Then I'll add it all together
8 + 1.6 = 9.6
120 + 9.6 = $129.6
If you can do this mentally then do this equation below
120 x 1.08 = $129.6