1. Zen problem
A Buddhist monk got an errand from his teacher: to meditate for exactly 45 minutes. He has no watch; instead he is given two incense, and he is told that each of those sticks would completely burn in 1 hour.  The sticks are not identical, and they burn with varying yet unknown rates (they are hand-made). This means that there is no guarantee that half of the stick will burn in 30 minutes. 
So he has these two incense sticks and some matches: can he arrange for exactly 45 minutes of meditation? 
Ans: He needs to count time in 30 minutes plus 15 minutes to get 45 minutes. While 30 minutes is half the time the stick takes to burn, 15 is quarter that. So just when he wishes to begin, he lights one stick on both ends and the second stick at one end. After half an hour, the first stick would have burnt out completely while the second will still be partially burnt and will have enough incense to burn for another half hour. He must now at once light the other end of the second stick so both ends begin to burn. Hence this stick will burn out in half of 30, that is, 15 minutes, after which he can stop meditating.

2. Bridge crossing
A group of four people has to cross a bridge. It is dark, and they have to light the path with a flashlight. No more than two people can cross the bridge simultaneously, and the group has only one flashlight. It takes different time for the people in the group to cross the bridge:
        Aisha crosses the bridge in 1 minute,
        Bindu crosses the bridge in 2 minutes,
        Caroline crosses the bridge in 5 minutes,
        Deena crosses the bridge in 10 minutes. 
How can the group cross the bridge in 17 minutes? 
Ans: Two people have to go across each time, with one flashlight and one person has to bring it back. Caroline and Deena are the slowest, so they should never have to return, otherwise the crossing will take too long. So only Aisha or Bindu should return. Also, if Caroline or Deena cross with any of the other two, they will slow them down to their speed, because both need the flashlight. So clearly it is best for Caroline and Deena to cross together. If you start by these two crossing, one of them will have to come back and that will take too long.
So let us start with Aisha and Bindu crossing, and (say) Aisha returns with the flashlight. Bindu has now crossed and is waiting for the others.  This takes 2 minutes up and 1 minute back, so 3 minutes in all.  Now Aisha hands over the flashlight so that Caroline and Deena can cross together as we have suggested. This takes 10 minutes, the time for the slower Deena to cross. So 13 minutes have passed. Now, as we said, neither Caroline or Deena should return, so Bindu has to cross back with the flashlight in 2 minutes (so 15 minutes have passed). Then Aisha and Bindu cross back together in 2 minutes, so they are now all across in 17 minutes.

3. Two mothers
Two mothers are sitting in a street cafe, talking about the children. One says that she has three daughters. The product of their ages equals 36 and the sum of the ages coincides with the number of the house across the street. The second woman replies that this information is not enough to figure out the age of each child. The first agrees and adds that the oldest daughter has the beautiful blue eyes. Then the second solves the puzzle. You might solve it too! 
Ans: There are three children. The product of the 3 ages is 36. The sum of the ages is the house number across the street which we are not given (but the person talking to the first mother) can see the number.  All possible ages of the children (starting with the oldest) that multiplyout to 36 are listed below, along with the sum of the ages. There are only 8 possibilities.
A. 36 + 1 + 1 = 38 
B. 18 + 2 + 1 = 21 
C. 12 + 3 + 1 = 16 
D. 9 + 4 + 1 = 14 
E. 6 + 6 + 1 = 13 
F. 9 + 2 + 2 = 13 
G. 6 + 3 + 2 = 11 
H. 4 + 3 + 3 = 10 
The second woman could see the house number across the street. Whichever sum matched that of the house number gives the ages of the children. But she said that there was not enough information to solve the puzzle. This is only possible if there is more than one set of solutions which add to the same number. We see that these are the solutions E and F, both adding to 13.  Therefore the house number across the street was 13. Now both options E and F allow for a set of twins (either 6 years old or 2 years old). When the first woman mentioned that her oldest daughter had blue eyes we can eliminate option E since here would be two eldest twins of the same age. So the answer has to be option F: the girl children ages are 9, 2, and 2.
Source: https://www.math.utah.edu/~cherk/puzzles.html