Subject: mathqa
Question: if ' a ' completes a piece of work in 3 days , which ' b ' completes it in 5 days and ' c ' takes 10 days to complete the same work . how long will they take to complete the work , if they work together ?
Option: 
A: 7 days
B: 9 days
C: 4.5 days
D: 1.5 days
Answer: D