Lin sold 4 more shirts than greg. fran sold 3 times as many shirts as lin. in total the three sold 51 shirts. which represents the number of shirts greg sold

 Your 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