(1) G + B = 1500
(2) G + R = 750
(3) G + B + R = 2000
Rearrange (1) to get B = 1500-G
Replace B in (3) to get G + (1500 - G) + R = 2000
Simplify to get R + 1500 = 2000, therefore R = 500
Insert R-value into (2) to get G = 250
Insert G-value into (1) to get B = 1250
