A can do a piece of work in 7 days of 9 hours each can do it in 6 days of7 hours each. How long will they to do it, working together hours a day?
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ANSWER : 3 days
STEP BY STEP EXPLAINED:
A can do in 7 days by 9 hours a day
i.e. 9 × 7 = 63 hours
B can do in 6 days by 7 hours a day
i.e. 7 × 6 = 42 hours
A's work of 1 hour = 1/63
B's work of 1 hour = 1/42
together their work = 1/63 + 1/42
= 105 / 63 ×42
= 5/ 126 work per hour
so together they take 126 /5 hours to complete the work
so 126 / 5 = 3 × ( 42 /5 )
= 3 × ( 1 ) days ---- ( 42/5 hours means one day )
= 3 days
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