A and B each working alone can do a work in 15 days and

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A and B each working alone can do a work in 15 days and 25 days, respectively. They started the work together but B left after some time and A finished the remaining work in 7 days. After how many days from the start did B leave?

1. 3
2. 5
3. 7
4. 9

Correct Option: B

Let B left work after N days from the start.
According to the question,
N/25 + (N + 7)/15 = 1
⇒ (3N + 5N + 35) / 75 = 1
⇒ 8N = 75 - 35
⇒ N = 40/8
∴ N = 5