Easy | General |

There is a light bulb inside a room and four switches outside. All switches are currently at off state and only one switch controls the light bulb. You may turn any number of switches on or off any number of times you want. How many times do you need to go into the room to figure out which switch controls the light bulb?

Hint

A lighted bulb also emits heat, and gets hot slowly (not instantaneously!)

Answer

1

Solution

I think this puzzle is stupid, but it was asked to my friend in an interview, so I can't skip it. The main point is that bulb gets hot slowly when turned on. Turn on bulb 1 and 2 keeping others off. Wait for 20 minutes. Switch off 2 and turn on 3, quickly enter room and touch the bulb. If the bulb is:

on and hot -> switch 1 controls it

off but hot -> 2

on but cool -> 3

off and cool -> 4

on and hot -> switch 1 controls it

off but hot -> 2

on but cool -> 3

off and cool -> 4

Source: Analytical Interview Puzzle

Enable Like and Comment Easy | General |

An airplane flies in a straight line from airport A to airport B, then back in a straight line from B to A. It travels with a constant engine speed and there is no wind. Will its travel time for the same round trip be greater, less or the same if, throughout both flights, at the same engine speed, a constant wind blows from A to B?

Answer

Wind will always slow down the journey!

Solution

The time during which the plane's speed is boosted is shorter than the time during which it is retarded, so the over-all effect is of retardation. Consider the example when plane's speed is same as wind. In one direction, plane's speed doubles, while in the other, it becomes zero! (i.e. it takes infinte time to finish the journey!)

Easy | General |

You are given two cords that both burn exactly one hour, not necessarily with constant speed. How should you light the cords in order to determine a time interval of exactly 15 minutes? Extra question: how to light just one cord and measure 15 minutes?

Hint

Because this lacks uniformity, we cannot break the cord in half, and expect it to burn in half the time. But we can burn both ends of a cord to finish it in half the time it would have taken otherwise!

Solution

Burn first cord at one end and the second cord at both ends. Half an hour later, second cord finishes burning. Now you can burn the other end of first cord, and it shall finish exactly in 15 minutes.

Now can you guess how to measure 15 minutes using only one cord?

Break the cord into approximately half, and burn these two cords form both their ends. If both these cords finish burning together, that means exactly 15 minutes have passed. If any one of these cords finishes first, break the other cords from approximately middle, and further burn all ends of these little cords. Continuing this way theoretically leads to exactly 15 minutes!

Now can you guess how to measure 15 minutes using only one cord?

Break the cord into approximately half, and burn these two cords form both their ends. If both these cords finish burning together, that means exactly 15 minutes have passed. If any one of these cords finishes first, break the other cords from approximately middle, and further burn all ends of these little cords. Continuing this way theoretically leads to exactly 15 minutes!

Source: Quant Interview

Enable Like and Comment Easy | General |

There are two beakers, one containing water, the other wine (equal volumes). A certain amount of water is transferred to the wine, then the same amount of the mixture is transferred back to the water. Is there now more water in the wine than there is wine in the water?

Answer

Equal!

Solution

The main point is that the final volumes of mixture liquids are equal. This means in jar A, volume of water + wine is same as that in jar B. Assume original volume as L. Suppose final volume in jar A is x for water and (L-x) for wine. This implies that jar B has x amount of wine and (L-x) of water. This proves that there are equal amounts of water in the wine jar and wine in the water jar.

Source: Common

Enable Like and Comment Easy | General |

Light bulbs are numbered 1 to 100, and kept off initially. First person comes and toggles all the bulbs which are multiple of 1, i.e. he switches all bulbs to on. Second person toggles all multiples of 2, i.e he turns of even bulbs. Third person comes and toggles all multiples of 3. This process continues till 100 persons pass. After this, how many bulbs are ON?

Hint

Consider the bulb number 9.

Answer

10 bulbs

Solution

We notice that for a perfect square (like 9), the number of factors are always odd, for example:

Number of factors of (16) = # [1,2,4,8,16] = 5

Note that for non-square numbers, factors are even.

As a factor toggles the state of a bulb, bulb number 9 will be toggled by 1,3 & 9. Thus bulb number 9 will switch ON, OFF, ON respectively. Note that odd number of factors cause bulb 9 to be ON at the end.

We note that for odd number of factors is the cause of bulb staying on at the end. Similarly every squared digit bulb will be switched on, and rest will remain off after all factors toggle. Thus the bulbs 1,4,9....81,100 are ON, at the end. Hence 10 bulbs are on.

Number of factors of (16) = # [1,2,4,8,16] = 5

Note that for non-square numbers, factors are even.

As a factor toggles the state of a bulb, bulb number 9 will be toggled by 1,3 & 9. Thus bulb number 9 will switch ON, OFF, ON respectively. Note that odd number of factors cause bulb 9 to be ON at the end.

We note that for odd number of factors is the cause of bulb staying on at the end. Similarly every squared digit bulb will be switched on, and rest will remain off after all factors toggle. Thus the bulbs 1,4,9....81,100 are ON, at the end. Hence 10 bulbs are on.

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