**Concept:** **The question tests on the basics of 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**