A a+b=431 2a+5b=1996
a=431-b 2(431-b)+5b=1996
a=431-b 862-2b+5b=1996
a=431-b 3b=1134
a=431-b b=378
a=431-378=53 b=378
B a+b=497 3a+7b=2791
a=497-b 3(497-b)+7b=2791
a=497-b 1491-3b+7b=2791
a=497-b 4b=1300
a=497-b b=325
a=172 b=325
C a+b=315 4a+7b=1875
a=315-b 4(315-b)+7b=1875
a=315-b 1260-4b+7b=1875
a=315-b 3b=615
a=315-b b=205
a=110 b=205
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