Question

Your friend has $100 when he goes to the fair. He spends $10 to enter the fair and $20 on food. Rides at the fair cost $2.00 per ride. Which function can be used to determine how much money he has left over after x rides?

Answer 1

You could use the function: 

f(x)=30+2x
100=30+2x 

In which lets say he rode 3 rides. Plug in 3 for x.

100=30+2(3)

Then Multiply 
100=30+6

Then combine 30 and 6 to get 36.
Then you can subtract 36 from 100 to get 
$64 left over.