Find how many coins actually increase or decrease when the coins are exchange into another value. That's how you find the number of 20¢ and 10¢ coins. For the 5¢ coins, use the value of $5 to deduct the 20¢ and 10¢ coins values to find the total value of the 5¢ coins. Then take the value divide by 5¢ to give you the no. Of 5¢ coins available.