How many books would need to be purchased from each club for the club's to cost the same

First club 
y= 40 + 10x              y= 40 + 10(8)                =120

Second club 
y= 15x                      y= 15(8)                         =120

now plug a number of the answer choices in each equation until you get the same result for both

The answer is C

One way could be 210 hundreds, 5 tens, and 4 ones. I will think of others but for now hope this helps