I'm not completely sure which way they would like for you to do it but anyways 3 to 15 is a times 5 so 5 times 5 is 25 so a possible width can be 25 as in 15 by 25 or 5 to 15 is times 3so 3 times 3 is 9so a possible width can be 9as in 15 by 9 I personally think 25 inches wide would be the better answer