Brainstorm

Logic Level 2

In front of you, there are 9 coins. They all look absolutely identical, but one of the coins is fake. However, you know that the fake coin is lighter than the rest, and in front of you is a balance scale. What is the least number of weightings you can use to find the counterfeit coin?

The answer is 2.

1 solution

Pratyush Pushkar
Jan 15, 2016

First, divide the coins into 3 equal piles. Place a pile on each side of the scale, leaving the remaining pile of 3 coins off the scale. If the scale does not tip, you know that the 6 coins on the scale are legitimate, and the counterfeit is in the pile in front of you. If the scale does tip, you know the counterfeit is in the pile on the side of the scale that raised up. Either way, put the 6 legitimate coins aside. Having only 3 coins left, put a coin on each side of the scale, leaving the third in front of you. The same process of elimination will find the counterfeit coin.

Technically the answer is $1$ . You can theoretically pick two coins at random and weigh them, by chance one of them could be fake and it took $1$ weighing to find it.

I understood what the question was asking so I did input $2$ .

I think it needs to be worded a little different.

Isaac Buckley - 5 years, 5 months ago

Thanks for the suggestion but suppose if there are gunny bags instead of the coins then how could you distinguish among them just by looking

Pratyush Pushkar - 5 years, 5 months ago

What Isaac means is that your question could be understood as, in the best case, how many weighing need to be done, and thus the answer could be 1

Tran Hieu - 5 years, 5 months ago

Brainstorm

The answer is 2.

1 solution

0 pending reports