If Bill and his friends go on both rides, their total cost will be $15.
To find the total cost of the rides, we need to add the cost of the roller coaster ride and the cost of the bumper car ride.
The cost of the roller coaster ride is $10. The cost of the bumper car ride is $5. Therefore, the total cost of the rides is:

Therefore, if Bill and his friends go on both rides, their total cost will be $15.
The following question may be like this:
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? The cost of the roller coaster ride is $10. The cost of the bumper car ride is $5.