Since, C alone completed the remaining work in 10 days. ∴ can complete the whole work in 10×10=100 days Now, work done by A,B and C together in 1 days =
1
20
+
1
25
+
1
100
=
5+4+1
100
=
10
100
=
1
10
∴ If they work together, they will complete the whole work in 10 days.