good, but that doesn't mean that dp's daughter would know that intuitively.
Well, let's see:
I order for the result to end with 5 (4095), one of the numbers you are looking for needs to end with 5.
What are the numbers between 50 and 70 that end with 5?
Only 55 and 65 fit the requirement.
The other one needs to end with 1 or 3 or 5 or 7 or 9.
so the list is: 51 53 55 57 59 61 63 65 67 69.
It will not take long for you to find out when multiplying one of the former by one of the latter..
Well, you know that at least one of the multipliers has to have a 5 in the one's place because your product is a multiple of 5. So one of your multipliers has to be either 55 or 65 because those are the only two whole numbers that fit the criteria between 50 and 70. So I divided 4095 by 65 and got 63.