Total slices required for children,
.
Total slices required for adults,

So, total slices required are, 20 + 80 = 100 .
Number of pizza, n = 100/8 = 12.5
So, he has to order 13 pizza cause pizza cannot be purchased in fraction.
Total price, P = 13 Ă— 9 = $117 .
Hence, this is the required solution.