The average number of a, B and C is 70, a: B = 2:3, B: C = 4:5, B number______ ．

A: B = 2:3 = 8:12, B: C = 4:5 = 12:15, so a: B: C = 8:12:15, 8 + 12 + 15 = 35, 70 × 3 = 210, 210 × 1235 = 72, a: B is 72

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