At the movie theatre, child admission is $6.00 and adult admission is $9.90 . on saturday, 155 tickets were sold for a total sales of $1312.20 . how many child tickets were sold that day?

57 children tickets were sold on Saturday.

You could set this up as a system of equations.
x + y = 155
6.00x + 9.90y = 1312.20

First step is multiplying the first equation by -6 to use the elimination method.

-6.00x + -6.00y = -930
6.00x + 9.90y = 1312.20

The positive and negative 6x cancel out until you are only left with the y.

Combining them would make your equation...
3.9y = 382.2

Next you would divide by 3.9 on both sides to get the y by itself.

The result of that would be y = 98. Which means 98 adult tickets were sold. Then you would plug y into either of the equations to solve for x.