It is given that
P,Q and
R together can complete the work in
6hrs.
P,Q and
R together worked for 2 hours, this implies they completed
1/3 rd of the work.
R left and remaining work is finished by
P and
Q in 7 hours.
Therefore,
P and
Q completed
2/3rd of the work in 7 hours.
This implies
P and
Q together can complete the work in 21/2 hours.
Let the total work be
LCM(6,21/2), i.e. 42 units.
P,Q and
R together can complete 7 units in one hour.
P and
Q together can complete 4 units in one hour.
This implies,
R can complete 3 units in one hour.
Time taken by
R alone to complete the work
==14 hours
The answer is option C.