Question 766778
First focus on either the radius of the circle or the side length of the square.  I will choose the circle's radius.  You know that later, you must deal with the sum of their perimeters.


Let r=radius of circle.  The square that perfectly is circumscribed around it has side 2r.  Draw this situation so you can visualize well.


"perimeter" or circumference of the circle is {{{2*pi*r}}}.
Perimeter of the square is {{{4*(2r)}}}.


The original wire length is 30 cm.  The total length of wire forming this circle and square must be the sum of their perimeters.


{{{highlight(2*pi*r+4(2*r)=30)}}}, find r.  THEN find {{{2*r}}} which is the diameter.


STEPS FROM THAT...
{{{r(2*pi+8)=30}}}
{{{r(pi+4)=15}}}
{{{r=15/(pi+4)}}}
..
.