Raising average scores from 87% to 90%

Question

I am looking into customer satisfaction surveys and out of 97 surveys this month, our store has a satisfaction rate of 87%. How many 100% surveys are needed to hit a 90% satisfaction rate this month?

Answer 1

[(97*,87)+n]/(97+n)=.9

(84.39+n)/(97+n)=.9

84.39+n=87.3+.9n

.1n=2.91

n=29.1