72/2=36, which is the square of 6.

Logic: divide each number by 2. It will be perfect square.

All perfect squares are there except 36.

Hence Answer is 72.

If

4+5+4+5=81 5+6+5+6=16 6+7+6+7=64 1+2+1+?=16

What is the missing number?

Find a number which is Divided by 2 the balance should be 1, div by 3 bal 2, div by 4 bal 3, div by 5 bal 4, div by 6 bal 5, div by 7 bal 0, div by 8 bal 7.