3. There are six numbers $a b, a(a+10), a(b+10), b(a+10)$, $b(b+10),(a+10)(b+10)$ for any pair of natural numbers $a$ and $b, a \neq b$. Among them, for some $a$ and $b$, perfect squares can be found. For which $a$ and $b$ will the number of perfect squares be maximized?
Answer: three squares when $a=1, b=k^{2}, k \neq 1, k \in Z$.