a + b = 8
a) x + 2 a + 2 b = 20
x + 2 ( a + b ) = 20
x + 2 · 8 = 20
x = 20 - 16
x = 4
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b) 5 a + 5 b - x = 12
5 ( a + b ) - x = 12
5 × 8 - x = 12
40 - x = 12
x = 40 - 12 ⇒ x = 28
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c) x + 6 = 23 + 7 a + 7 b
x + 6 = 23 + 7 ( a + b )
x = 23 - 6 + 7 × 8
x = 17 + 56
x = 73