• Bacteria in a bottle - Bacteria multiples every minute. You place a bacteria into an empty cup at 12. By 1pm, it’s full. At what time was it half full?

Solution = 12:59pm

• Poison Bottle & Rats - There are 8 bottles and 3 rats. One bottle is poisoned. Once poisoned each rat has 5 minutes to live. Within 5 min, how can you use all 3 rats to figure out which bottle was poisoned.

Solution

1
2
3
4
5
6
7
8

Bottle 7 - 1 1 1 - R1, R2, R3 rats drank off of this bottle
Bottle 6 - 1 1 0 - R1, R2 rats drank off of this bottle
Bottle 5 - 1 0 1 - R1, R3 rats drank off of this bottle
Bottle 4 - 1 0 0 - R1 rats drank off of this bottle 
Bottle 3 - 0 1 1 - R2, R3 rats drank off of this bottle
Bottle 2 - 0 1 0 - R2 rats drank off of this bottle
Bottle 1 - 0 0 1 - R3 rat drank off of this bottle
Bottle 0 - 0 0 0 - None drank off of this bottle

Now if Bottles 1-7 contain poison, then the killing of rats combination will tell us the answer. And If no rat gets killed, Bottle 0 has the poison.

• Identify heavier ball out of 8 - One ball is slightly heavier out of 8 balls. How many times do you need to use a scale to determine which is heavier?

Solution - 2 steps max Break the balls into the following groups: (1,2,3),(4,5,6),(7,8)

Step 1: Weigh (1,2,3) against (4,5,6)

Case A: If both are equal, in step 2, compare 7 and 8 Case B: If one of (1,2,3) or (4,5,6), then in step 2, out of the heavier group, take 2 out of 3 and weigh again.

• Light switch combination - There are 10 light switches. How many different combinations can you have of on/off?

Solution: 2 states: on and off. So, 2^10 combinations

• ** - There are 2 cars at a railroad track 100 miles apart. Each car is moving at 10mph towards each other and there is a flying bird that flies back and forth at 20mph and turns around whenever it hits a car. How far did the car travel before it hit both trains?