A can complete work in 25 days and B can complete the same work in 20 days. They started the work together but B left after 4 days and A continued to work. In how many days will the entire work be completed?

In 20 days the entire days would be completed, Explanation: Let us assume total worked to be done is 100 units (LCM of 20 and 25), so A can complete 4 units per day and B can do 5 units per day. They started together and worked for 4 days together hence work done by A and B is (4+5)*4=36 units and hence remaining job is 100-36=64 units now this will be done by A only since A can complete 4 units in a day, hence he can complete 64 units in 16 days. Now question ask us about days in which entire work will be completed,which is equal to 16+4=20 day

A's 1 day work- 1/25 B's 1 day work- 1/20 (A+B)'s 1 day work- 1/25 + 1/20 = 9/100 (A+B)'s 4 days work- 9/100 * 4= 36/100 Work left after 4 days- 1 - 36/100 = 64/100 B left, hence, A will complete the rest of the work in= 64/100 divided by 1/25 = 16 days A will take another 16 days to complete the work.(Total days- 4+16= 20) If the question is asking about how many more days A will take then answer is 16, whereas if it is talking about total days then answer is 20.

20