Since this is an estimate, you need to do it in your head. The way I did it was I divided 40 by 5 since 20% is one fifth of 100%, then I divided 2 by 5 and added them together which equaled 8.40.
Move decimal point one place to the left (making $4.20), then double that answer (making $8.40). You said estimate so I rounded down to $8 but a more precise answer would be $8.40.
