The two coins you have are a quarter (25 cents) and a nickel (5 cents).
At first glance, it might seem tricky since the question specifies that one of the coins is not a nickel. But in reality, the coin that isn’t a nickel is the quarter. The other coin is indeed a nickel, which allows the total to equal 30 cents. This riddle plays on the wording, leading you to think both coins can’t be nickels when in fact only one of them is.