Asmall fair changes an admission fee for children and adults. the cost for admission is $3 per adult and $2 per child. on a certain day, total sales were $900, and 350 people attended the fair. how many children attended the fair that day?

We need to make a system of equations. A=Adults and C=Children.
A+C=350; this means how many people are there.
3A+2C=900; this deals with money.
We need to eliminate a variable, let's eliminate C by multiplying the first equation by -2.
-2a-2C=-700
3A+2C=900
A=200
This means there was 200 adults, which means there had to be 150 children.

Provide an equation