Which is the smallest number that can be subtracted from 4000 to form a perfect square?

If we calculate the square root of 4000 = 63.24

The whole number smaller than 63.24 is 63

Now,

63² =3969

3969 is a perfect square.

So, the least number which is to be subtracted from 4000 to make it a perfect square= 4000−3969=31.