arithmetic - rupa can do a piece of work in 7 days of 9 hours each and harini can do it in 6 days of 7 hours each how long will they take to do it working together 825 hours a day - skillgun

Let A = Rupa, B = Harini.
A can complete the work in (7 * 9) hrs = 63 hrs.
B can complete the work in (6 * 7) hrs = 42 hrs.
As 1 hours work = 1/63 and B's 1 hours work = 1/42
(A+B)s 1 hours work = [(1/63) + (1/42)] = 5/126
Both will finish the work in (126/5) hrs
Number of days of 8*(2/5) hrs each = (126/5) * (5/42) = 3 days.