O T T F F



What are the next two letters in this sequence?

O T T F F _ _

Ah! Click here to read the solution.



The Three Bulbs



There are three light bulbs on the tenth floor of a building and there are three switches on the ground floor which are used to turn on the light bulbs (one switch turns on one bulb).
A man is on the ground floor and he knows that the three light bulbs on the tenth floor are all turned off.

How can he just goes up to the tenth floor only ONCE to determine which switches control which light bulbs?

Ah! Click here to read the solution.