Question 544405
It isn't, exactly!
If the square has an area of 100, then the side of the square must be 10 (A = 10*10 = 100).
The inscribed circle must have a radius equal to half the length of the side of the square (10/2 = 5
The area of a circle whose radius is 5 is: 
{{{A = (pi)r^2}}}

{{{A = (pi)5^2}}}
{{{A = (3.14)(25)}}}
{{{A = 78.5}}} Approximately.