Correct option is B
Given: MI alone: 14 days, M2 alone: 70 days, M1 + M2 + M3 together: 7 days
Formula Used: Work done per day = 1/Days for each worker
Solution:
Work done by M1 in 1 day = 1/14,
M2 in 1 day = 1/70,
M1 + M2 + M3 in 1 day = 1/7
Let work done by M3 in 1 day = 1/M3 days
141+701+M31=71705+701+M31=71706+M31=71M31=71−706M31=7010−706M31=704M31=352M3=235=17.5days