Chapter 10: Brainteasers 357
that if you multiply their ages, the result is 36. He says
he needs more info so she tells him that the total of
their ages is the address of the building next door. He
goes and looks, then comes back and says he still needs
more information. She tells him that she won’t answer
any more questions because her eldest child is sleeping
upstairs and she doesn’t want to wake him.
What are the children’s ages?
(Thanks to tristanreid.)
Ants on a circle You have a circle with a number of ants
scattered around it at distinct points. Each ant starts
walking at the same speed but in possibly different
directions, either clockwise or anticlockwise. When two
ants meet they immediately change directions, and then
continue with the same speed as before. Will the ants
ever, simultaneously, be in the same positions as when
they started out?
(Thanks to OMD.)
Four switches and a lightbulb Outside a room there are four
switches, and in the room there is a lightbulb. One of
the switches controls the light. Your task is to find out
which one. You cannot see the bulb or whether it is on
or off from outside the room. You may turn any number
of switches on or off, any number of times you want.
But you may only enter the room once.
(Thanks to Tomfr.)
Turnover In a dark room there is a table, and on this
table there are 52 cards, 19 face up, 33 face down. Your
task is to divide the cards into two groups, such that in
each group there must be the same number of face up
cards. You can’t switch on a light, ask a friend for help,
all the usual disalloweds. Is this even possible?
(Thanks to golftango and Bruno Dupire.)