R alone can do a work in 10 days working 3 hours a day. S alone can do the same work in 6 days working 5 hours a day. How many hours do they need to work together in a day to complete the same work in 4 days?
Show answer & explanation
Correct answer: C
Explanation
R's total work time = 10 × 3 = 30 hours
R's hourly work efficiency = 1/30
S's total work time = 6 × 5 = 30 hours
S's hourly work efficiency = 1/30
Combined work efficiency of both = 1/30 + 1/30
= 2/30 = 1/15
To complete the work in 4 days -
Total hours = 4 × 15/2 = 30/2 = 3.75 hours