Giải:
Ta có: `2a=3b => a/3 = b/2 => a/21 = b/14`
`5b=7c => b/7 = c/5 => b/14 = c/10`
`=> a/21 = b/14 = c/10`
Gọi `a/21 = b/14 = c/10 = k`
`=> a = 21k ; b = 14k ; c = 10k`
`=> a - b + c = 21k - 14k + 10k = 51`
`=> k (21 - 14 + 10) = 51`
`=> k . 17 = 51`
`=> k = 3`
`=> a/21 = 3 => a = 63`
`=> b/14 = 3 => b = 42`
`=> c/10 = 3 => c = 30`
Vậy ...................
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