<p>@yujin623, the reason it is multiplied by 3 is because there are 3 different ways to get a 5 two times. You could have 5 5 another # , 5 another # 5 , or another # 5 5. So those 3 options are plausible so you need to multiply getting 2 5s in one way by 3. Get what I mean?</p>