Subject: mathqa
Question: a , b and c can do a piece of work in 14 , 30 and 40 days respectively . they start the work together but c leaves 4 days before the completion of the work . in how many days is the work done ?
Option: 
A: 7.6 days
B: 8.7 days
C: 7.7 days
D: 5.7 days
Answer: C