How much was the original purchase if a 2% service charge is added to the price and the total is $4200.

I know the answer is 4117.65 with a fee of 82.35. I did this by guess and check to get the answer. I tried a proportion

x/4,200 = 2/100 but that didn't work. I get $84 I know why this doesn't work but used to get value to start at.

How can I solve this with out guess and check?

Thanks in advance.