Bill and his friends want to ride the roller coaster and the bumper cars. if they go on both rides, what will be their total cost?

Is there any more information given? you need to add up the cost of each ticket, so for instance if each ticket for the roller coaster is $10, they would need $20 so they can get two tickets so the both of them can get on... and if the tickets for the bumper cars are $5 each, they would need $10 so they can get two tickets so they both can get on the bumper cars as well... 
Just apply this equation when you figure out how much each ticket costs.

x= cost of the ticket
y= total cost

x(2+2) = y