Your Answer: D
Why....
You can make equations out of the information
Let L be Lin, G be Greg, and F be Fran
L = 4 + G ---"Lin sold 4 more shirts than Greg"
F = 3L ---"Fran sold 3 times as many shirts as Lin"
F + G + L = 51 ---"In total the three sold 51 shirts"
Use F + G + L = 51
Substitute the equations in for F and L (because you need to know the G) like this....
(3L) + G + (4+G) = 51
You still have a variable besides G in there... you can use the L= 4+G and substitute again so that there are only G's
( 3(4+G) ) + G + (4+G) = 51 ---- SIMPLIFY :D
( 12 + 3G ) + G + 4 + G = 51
12 + 3G + G + 4 + G = 51 ---Combine like terms
16 + 5G = 51