Q: The H.C.F. of two numbers is 12 and their L.C.M. is 600. If one of the numbers is 720, then the other is:
Options:
(A) 8
(B) 10
(C) 12
(D) 4
(E) 6
A: OK... Stream of consciousness: Other number = (12 x 600)/720 = 10.
The answer is (C).

Q: A man rows his boat 78 km downstream and 56 km upstream, taking 4 hours each time. Find the speed of the stream?
Options:
(A) 6
(B) 5
(C) 7
(D) 8
(E) 3
A: OK... Stream of consciousness: Speed downstream = d/t = 78/(4) = 20 kmph
Speed upstream = d/t = 56/(4) = 14 kmph
The speed of the stream = (20 - 14)/2 = 3 kmph
The answer is (E).

Q: One copy machine can make 20 copies a minute, and a second copy machine makes 10 copies a minute. If the two copiers work together, how long would it take them to make 900 copies?
Options:
(A) 10 minutes
(B) 20 minutes
(C) 25 minutes
(D) 30 minutes
(E) 35 minutes
A: OK... Stream of consciousness: Total work done by both machines in a minute = 20 + 10 = 30 copies
Total number of copies required = 900
Time = 900/30 = 30 mins
The answer is (D).

Q: Three pipes A, B and C can fill a tank from empty to full in 30 minutes, 20 minutes and 10 minutes respectively. When the tank is empty, all the three pipes are opened. A, B and C discharge chemical solutions P, Q and R respectively. What is the proportion of solution R in the liquid in the tank after 3 minutes?
Options:
(A) 5/11
(B) 6/11
(C) 7/11
(D) 8/11
(E) 9/11
A:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
OK... Stream of consciousness: Part filled by (A + B + C) in 3 minutes = 3(1/30 + 1/20 + 1/10) = 11/20
Part filled by C in 3 minutes = 3/10
Required ratio = 3/10 * 20/11 = 6/11
The answer is (B).