Martin finds a shirt on sale for 10% off at a department store. the original price was $20 martin must also pay 8.5% sales tax. how much is the shirt before taxes are applied?

Normally it is
cost times discount, then the discounted price times sales tax

so since we want before tax is applied, we disregard the 8.5% and find the 10% off

10% off means 90% (100-10=90)

so 90%=90/100=9/10
9/10 of $20
'of' means multiply with fractions
9/10 times 20=180/10=18
the shirt is $18 before tax