At the movie theatre, child admission is $5.40 and adult admission is $8.70 . on friday, 159 tickets were sold for a total sales of $1079.70 . how many adult tickets were sold that day?

Alright this is gonna be a long one

Lets called child tickets x and adult tickets y 

so x + y should be equal to 159 since they said that the total sum of the tickets is 159 
so thats your first equation : x + y = 159

next form an equation using the prices, 
5.40x + 8.70y = 1079.70

now that you have your two equations, just substitute the first into the second (you could either substitute for x or y by saying x = 159-y or that y=159-x)

Hope this helps.

I’m vary good with science if you need with that