Number Puzzles

Here is another my fav math book, I bought it in Kinokuniya a long time ago, he3.

Quite interesting puzzles inside here and a little bit tricky too.

Let say, in page 23, we have Coins puzzle.


Here is the puzzle:

" Two coins total 15 cents. One of them is not a 5-cent coin. What are these coins? (There are only 5, 10, 20 and 50 cent coints in the country) "



so, can you answer that?