Answer:
10.672hrs ( maybe)
Explanation:
A can complete the work in 20 hrs then 1/3 part can be completed in 20*1/3=6.67hrs
B can complete the 1/4 part of the work in same time then B can complete hole work in 6.67*4=26.68 hrs
B can complete the 1/5 part of work in 26.67/5=5.336hrs
according to question C can complete 1/2part of work in same time then the whole work can complete in
2*5.336=10.672hrs