You are considering buying 9 silver coins that look alike, but you have been told that one of the coins is a lightweight counterfeit. What is the least number of weighing on a balance scale that you can use to be certain that you have found the counterfeit coin?

I know how to do the weighing, but rusty on the math.

Weigh 8(a) and 1(b)
If weight of a/8 <> b then b is the bad coin

Split 8 into two piles of 4(c) and 4(d)
If weight of c/4 <> b then pile c contains the bad coin
If weight d/4 <> b then d contains the bad coin

Take the bad coin pile and divide in two piles of 2
Do the same test.

Then you'll have a pile of 2
Weigh each coin and compare to b

Hi, niceanne!

I'm not sure what you mean by "do my homework too". Would you mind explaining that?

Thanks!