IBM Ponder This #261 · January 2020

A gardener has 12 barrels of special fluid to water his orchids; exactly two of the barrels are poisonous. Using any amount, even a single drop, from a poisonous barrel will kill the precious orchid in 24 hours. The gardener has only three days before the orchids die unless he can figure out which ten barrels are the good ones. He is willing to sacrifice four orchids by watering them using a mix from a subset of these barrels. How can he find the poisonous barrels?

For example, for eight barrels labeled A, B, C, D, E, F, G, and H, only two days are enough:

	Day1	Day2
1	B	DG
2	DF	AB
3	AH	CF
4	C	GH

In the first day he waters orchid #1 from barrel B, on the second day he uses a mix from barrels D and G; etc. Supply your answer in the format B.DG; DF.AB; AH.CF; C.GH (corresponding the solution to the example above).

Solution

Best opened after a real attempt

To be added.