Question 204772
In another solution to this problem there are some errors (which are probably typing errors). So I am going to provide a correct solution, based on the otherwise correct solution.<br>

The diameter of the circle is also the side of the square
:
Circumference (C) + perimeter of the square (P) = 30
{{{C = pi*d}}}
{{{P = 4s}}}
;
Let x = the diameter and the side of the square
;
Substituting these into the equations
{{{pi*x + 4x = 30}}}
{{{(pi + 4)x = 30}}}
{{{x = 30/(pi + 4)}}}
Using the approximation {{{pi = 3.14159}}} we get
{{{x = 30/7.14159}}}
{{{x = 4.2}}} approximately.