Quantitative Aptitude :: Time and Work - Discussion

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Explanation:
find the (P + Q + R)'s 1 hour's work = (1/8+1/10+1/12) = 37/120

So Work done by P, Q and R in 2 hours = (2*37/120) = 37/60

Now find the Remaining work = (1?37/60) = 23/60

(Q + R)'s 1 hour's work = (1/10+1/12) = 11/60
OR
Time taken to do 11/60 part of work by (Q+R) = 1 hrs.
=> so to do whole work by (Q&R) = 60/11 hrs
so for 23/60 part of work = (23/60*60/11) hrs. = 23/11 = ~2 hrs.

So, the work will be finished approximately 2 hours after 11 a.m . i.e., around 1 p.m.