Concept: The question tests on Rates(Time and Work)
Solution:
Let the time taken by Z = a days
Work done by X in 1 day = 1/15
Work done by Y in 1 day = 1/10
Work done by Z in 1 day = 1/a
Work done by X, Y and Z in 1 day = 1/5
Therefore 1/15 + 1/10 + 1/a = 1/5
1/a = 1/5 - 1/10 - 1/15 = (6 - 3 - 2) / 30 = 1/30
Z alone finishes the work in 30 days
Ratio of Work done = 1/15 : 1/10 : 1/30 = 2 : 3 : 1
Amount paid to Z = (1 / 6) * 720 = 120
Option D