Assume the average weight initially was W kg.
So total weight of 25 persons = 25W
Let weight of the man who leaves = W1
The remaining weight of 24 persons = 25 W - W1
Total weight after new man comes in = 25 W - W1 + 70
New average weight = (25 W - W1 +70) / 25 = W + 1 {Given that average weight is increased by one }
25W - W1 +70 = 25 W + 25
W1 = 70 - 25 = 45 Kg
Hence the weight of the man who is going is 45 kg.