The answer is 20n + 10 = 90 because the bar seats 10 people so you take 90-10 and you're left with 80. Four seats are at a table so you take 80/4 and you're left with 20. All you need to do now is reverse the order to fit the answer, and since you originally subtracted, you'd add. You could also do 20x4 + 10 and see that it equals 90 to check your answer.
