The least coins to pay back would be one 35 cent coin, but there isn't such a coin. But we can use 2 coins: one 25 cents, and one of 10 cents: so the smallest amount is 2 coins. 

the largest amount of coins is achieved by using the coins with the smallest value: this would be the 5-cent-coins. so we'd need 35/7=7; 7 such coins

so the different is between 7 and 2 coins, that is 5 coins difference!
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