Răspuns :
a + b = 240
b + c = 360
7a + 10b + 3c = 7a + 7b + 3b + 3c = 7(a + b) + 3(b + c) = 7*240 + 3*360 = 1680 + 1080 = 2760
a + b = 240
7a + 7b = 1680
b + c = 360
3b + 3c = 1080
Adunam relatiile
7a + 7b + 3b + 3c = 1680 + 1080
7a + 10b + 3c = 2760