Sonya buys four pairs of shoes on sale for buy one, get one 50% off. The sales tax is 6.5%. If the original price for each pair of shoes was $35, how much does Sonya pay for the four pairs altogether?

50% of $35 is $17.50

So multiply $17.50 by 4 because she bought 4 pairs. Then take the answer, which is $70 and mutiply that by 0.065 (6.5 sales tax) and you will get $4.55 then take that away from $70 and you will get $65.45 which is how much she spent all together.