Algerbra Question

[MathStudent1999]

70. If A and B are natural numbers, then several of the digits from 0-9 could never be the units digit of (a^2)(b^2). What is the sum of the digits from 0-9 inclusive that could never be the units digit?

I thought of trying out each number one by one, but that would be too slow. I can't see what I should do now. Can someone help me?

 

[MathStudent1999]

Why in heck are you asking for the sum?
2,3,7,8

I tried the question again and also got 2,3,7,8. Adding them up, I got 20. Thanks!