Explanation:
100% = $85
1% = 100%/100 = 85/100 = $0.85
20% = 1%×20 = 0.85×20 = $17
so, the total cost of $85 + $17 = $102
which we could have gotten directly by multiplying
$85 by 1.2 (1.2 = 1 for the whole + 0.2 for 20%) = $102
but I wanted to show you the detailed, basic line of thinking behind the % calculations first.
whenever you have no other idea, get to 1% first and then take it from there.