A needs 80 (10 × 8) hours to complete the work alone. Similarly, B and C need 120 (15 × 8) hours and 60 (20 × 8) hours, respectively. A and B work for 36 (6 × 6) hours together. So, work done by them = 36 (
1
80
+
1
120
)+
H
160
= 1, where H is the number of hours C has to work to complete the remaining work. ∴ H = 40 hours So, C needs 4 days if he works 10 hours a day.