Answer with Explanation:
(P + Q + R)'s 1 hour's work = ( 1/8 + 1/10 + 1/12 ) = 37/120 .
Work done by P, Q and R in 2 hours =(37/120 x 2 ) = 37/60 .
Remaining work = (1 -37/60) = 23/60 .
(Q + R)'s 1 hour's work = (1/10 + 1/12) = 11/60 .
Now,11/60 work is done by Q and R in 1 hour.
So,23/60 work will be done by Q and R in (60/11 x 23/60 ) = 23/11 hours = 2 hours.
So, the work will be finished approximately 2 hours after 11 A.M., i.e., around 1 P.M.