Explanation:
a + b + c + d = 279
a = b
c = 2×(a + b) = 2×(2a) = 4a (because a = b)
d = 3×b = 3×a = 3a
so, the first equation looks like
a + a + 4a + 3a = 279
9a = 279
a = 31
b = 31
c = 4a = 4×31 = 124
d = 3a = 3×31 = 93
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