3 hrs. 45 min.

C

Split the pack to two parts of 25 and 15 cards. Now, consider the 15-card deck. Let there be ‘X’ face-up cards in this set (‘X’ may be 0 – 15).  Since total number of face-up cards in the pack is 15,  out of which ‘X’ are in now the 15-card deck,  the other deck (of 25) will have the remaining (15 – X) face-up cards. Now, I flip ALL cards in the 15-card deck. Since there were ‘X’ cards face-up earlier, now I have those X cards face-down and remaining (15 – X) cards face-up, which is equal to the No. of face-up cards in the other  deck. Solved.

0.25

The large inlet can fill the tank in 3 hrs., which means that, in 1 hr., it can fill 1/3 of the tank. Similarly, the smaller inlet takes 6 hrs. to fill the tank, which means it can fill 1/6 of the tank in one Hr. Thus, if both inlets are opened at the same time, it will fill 1/3 + 1/6 = 1/2 of the tank in 1 hr. However, the outlet will empty the tank in 9 hrs., which means that, in 1 hr., it will empty 1/9 of the tank. Thus, if all 3 pipes are opened simultaneously, the net effect in one hr. will be that (1/2 – 1/9) of the tank will be filled.

Hence, the portion of the tank that will be filled in in 0.64 hrs. will be (1/2- 1/9) * 0.64 = 0.2488… ~ 0.25

Since value of F is not given, no single answer is possible. There are an infinite number of possible answers.

One ‘combination’ answer is: D + F = 2A + B + 2C. There can be many such combinations.

Equally important?

181

Numbers increase by 11, 33, 55… Next increment should be 77. 104 + 77 = 181.

N

One, Two, Three, Four… Nine

A parachute that failed to deploy…

Yet another repeat, for the umpteenth time!

Lock the ring in a box and send it to the intended recipient. The recipient (who doesn’t have the key to that lock) shall affix one of his / her own locks to the package and send it back. The original sender, upon receipt, shall open and remove his own lock with its key so that only the second lock remains on the package, and once again send it across to the original recipient, who upon receipt, can open the lock for which (s)he has the key and get the ring.