Martin finds a shirt on sale for 10% off at a store. The original price was $20. Martin must pay 8.5%. How much is the shirt after sales taxes are applied?

I take it you meant, he's going to pay 8.5% in sales tax.

whatever% of anything is just (whatever/100) * anything.

thus, 10% of 20 is (10/100) * 20, or just 2 bucks.

so, the shirt was 20, but went down by 2 bucks, so is 18 then.

now, here cometh the taxman, 8.5% on that 18 is? well, yes, you guessed it, (8.5/100) * 18, or 1.53.

so, Martin ended up paying 18 + 1.53.