y = number of ferris wheel rides taken
x = number of roller coaster rides taken
it costs 2 tickets to go on the ferris wheel, so 2y represents how many tickets you use up when you ride on the ferris wheel y times
Similarly, 4x represents how many tickets you use up when you ride on the roller coaster x times.
2y+4x is the total number of tickets used up. We have 16 tickets, so 2y+4x = 16
Solve for y
2y+4x = 16
2y = -4x+16
y = (-4x+16)/2
y = (-4x)/2 + 16/2
y = -2x + 8
This equation lets you figure out how many times you can go on the ferris wheel if you know how many times you have gone on the roller coaster.
For instance, if you go on the roller coaster one time, then x = 1 and
y = -2x+8
y = -2(1) + 8
y = -2+8
y = 6
meaning you can go on the ferris wheel 6 times.