Question:
A drinks machine offers three selections - Tea, Coffee or Random but the machine has been wired up wrongly so that each button does not give what it claims. If each drink costs 50p, how much minimum money do you have to put into the machine to work out which button gives which selection?
(A) 5 rupees
(B) 10 rupees
(C) 25 rupees
(D) Infinity
Answer: A) 5 rupees
Explaination:
Each button does not give what it claims. So tea button may give coffee or random (either tea or coffee) and coffee bottom may give tea or random (either tea or coffee).
The random bottom should give either coffee or tea. If we start with this random button with one 5 rupees coin, then it will work for either tea button providing tea or for coffee button providing coffee.
If the random button works for tea, then the tea button must be for coffee or random. As the already random button is for tea, the coffee button cannot be tea, and hence random.
Thus,
random = tea, coffee = random, tea = coffee.
Similarly, if the random button gives coffee while we put 5 rupees coin, then
random
= coffee, tea = random, coffee = tea
Thus by 1st chance only we can found out which button works for which
selection with minimum money of 5 rupees only
