Help with College algebra question.

Question

At the movie theatre, child admission is $6.00

 and adult admission is $9.10

. On Wednesday, four times as many adult tickets as child tickets were sold, for a total sales of $890.40

. How many child tickets were sold that day? 

Answer 1

6C + 9.1A + 890.4

A=4C

6C +9.1(4C) = 890.4

42.4C = 890.4

C = 890.4/42.4 = 21 child tickets

A=4x21=84 adult tickets