Riddles, brain puzzles and mathematical problems

[Oscar House]

Make 3 piles of 9 coins each. Compare two of them and you'll know which of the piles contains the lighter coin. Divide that pile into 3 piles of 3 coins, then compare two of those. Now you'll know which of the 3-coin piles contains the lighter coin. Now just weigh two of the remaining coins and you'll find the right coin.

Υou are very close! Specify what happens if the scale balances in each case and you are good to go.

Right. If there's three piles to compare and two of the piles are the same weight (i.e. the scale balances), that means the lighter coin is in the third, unweighed coin pile and you'll just continue dividing that pile into three smaller piles until you find it.