Which one of the following is the minimum sum of $2$ integers whose product is $64$?

Let the two positive integers be $a$ and $b$ such that $a\times b=64$.
Let us find the pairs of positive integers whose product is $64$ and calculate their sum:

* $1\times 64=64⟹1+64=65$
* $2\times 32=64⟹2+32=34$
* $4\times 16=64⟹4+16=20$
* $8\times 8=64⟹8+8=16$

The minimum sum among the possible pairs is $16$.