L + F + G = 51
F = 3L...F = 3(G + 4)....F = 3G + 12
L = G + 4
(G + 4) + (3G + 12) + G = 51
5G + 16 = 51
5G = 51 - 16
5G = 35
G = 35/5
G = 7 <=== Greg sold 7
L = G + 4...L = 7 + 4....L = 11 <== Lin sold 11
F = 3G + 12.....F = 3(7) + 21....F = 21 + 12....F = 33 <== Fran sold 33