question:X can finish a work in 21 days. Y can finish the same work in 15 days. Yworked for 10 days and left the job. how many days does X alone need to finish the remaining work?
Options:
(A) 3
(B) 5
(C) 7
(D) 8
(E) 9
answer:(C)
Stream of consciousness:Work done by X in 1 day = 1/21
Work done by Y in 1 day = 1/15
Work done by Y in 10 days = 10/15 = 2/3
Remaining work = 1 2/3 = 1/3
Number of days in which X can finish the remaining work = (1/3) / (1/21) = 7

question:At 15:00 there were 21 students in the computer lab. At 15:03 and every three minutes after that, 3 students entered the lab. If at 15:10 and every ten minutes after that 8 students left the lab, how many students were in the computer lab at 15:44 ?
Options:
(A) 7
(B) 14
(C) 25
(D) 27
(E) 30
answer:(D)
Stream of consciousness:Initial no of students + 3 * (1 + No of possible 3 minute intervals between 15:03 and 15:44) -
8 *(1 + No of possible 10 minute intervals between 15:10 and 15:44)
20 + 3*14 -8 * 4 = 27

question:A and B complete a work in 9 days. A alone can do it in 18 days. If both together can do the work in how many days?
Options:
(A) 6 days
(B) 8 days
(C) 5 days
(D) 7 days
(E) 15 days
answer:
(A)
Stream of consciousness:1/9 + 1/18 = 1/6
6/1 = 6 days