21. A can do a work in 7 days of 9 hours each and B can do it in 6 days of 7 hours each. How long will they take to do it working together 8 2/5 hours a day?

A. 3 days B. 4 days C. 6 days D. 2 days

Answer Answer: Option A Explanation: A can complete the work in = ( 7*9) = 63 hours
B can complete the work in = ( 6*7) = 42 hours
A's 1 hour's work = 1/63 and B's 1 hour's work = 1/42
(A + B)'s 1 hour's work = (1/63 + 1/42) = 5/126 = Number of days of 8 2/5 hours each = (126/5 * 5/42) = 3 days

22. A does half as much work as B in three-fourth of the time. If together they take 18 days to complete the work, how much time shall B take to do it?

A. 30 days B. 35 days C. 40 days D. none of these

Answer Answer: Option A Explanation: Suppose B takes x days to do the work.
therefore A takes (2*3/4 x) = 3x/2 days to do it.
(A+B)'s 1 day's work = 1/18.
therefore 1/x + 2/3x = 1/18 or x = 30.

23. Worker A takes 8 Hours to do a job. Worker B Takes 10 Hours to do the same job. How long should it take both A & B working together but independently to do the Same Job?

A. 32/9 B. 40/9 C. 35/8 D. 30/5

Answer Answer: Option B Explanation: A's 1 Hour work=1/8, B's 1hour work =1/10
(A+B)'s 1 hour work =(1/8+1/10)=9/40.
Both A & B Complete the work in 40/9 days

24. A takes twice as much time as B or thrice as much time to finish a piece of work. Working together, they can finish the work in 2 days. B can do the work alone in

A. 4 days B. 6 days C. 8 days D. 12 days

Answer Answer: Option B Explanation: Suppose A,B and C take x, x/2 and x/3 hours respectively to finish the work.
Then, (1/x+ 2/x+3/x) = 1/2 = x = 12.
so, B takes 6 hours to finish the work.

25. A and B can together finish a work in 30 days. They worked together for 20 days and then B left. After another 20 days, A finished the remaining work. In how many days A alone can finish the job?

A. 40 B. 50 C. 54 D. 60

Answer Answer: Option D Explanation: (A+B)'s 20 days work = (1/30*20) = 2/3. Remaining work = (1- 2/3) = 1/3.
Now, 1/3 work is done by A in 20 days.
Whole work will be done by A in (20*3) = 60 days.