I wanted to buy a toy. I had four identical coins but these were not enough to pay for what I wanted, so I gave the shopkeeper a $10 note. He gave me $1 change. If I had had that coin before I could have paid the exact amount.
What were the coins I had before I bought the toy?
Solution:
The shopkeeper gave me $1 change from my $10 note, so the toy was worth $9.
$8 + $1 is $9, so I must have had four $2 coins.
Extension:
Would the answer change if I had told you that the shopkeeper had given me a single coin change but not told you what it was?