Correct option is D
Solution:
One day work of P and Q = 1/30
One day work of Q and R = 1/24
One day work of R and P = 1/20
One day work of P, Q and R = 1/2 (1/30+ 1/24+ 1/20)
= 1/2 ((4+5+6)/120)
= 1/2 (15/120)
= 1/2 (1/8)
=1/16
Therefore, P, Q and R can complete the work together in 16 days.