Hi. I am assuming that you are asking the price of the comic book after the 20% mock-up. Is that correct?
If yes, you can simply change the percent to a decimal and then multiply it by the original price to see the price of the markup. Once you have that answer, you just add to the original cost.
Example: $3.95 x .20 = $0.79
$3.95 + $0.79 = $4.74
I hope this helps.
Take care,
Diana