10 a - 20 b + 300 = 420
10 a - 20 b = 420 - 300
10 ( a - 2 b ) = 120
a - 2 b = 120 : 10
a - 2 b = 12
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2 a - 4 b = 2 × ( a - 2 b ) = 2 × 12 = 24
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6 a - 12 b = 6 × ( a - 2 b ) = 6 × 12 = 72
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