Question 844584
see http://www.algebra.com/algebra/homework/word/geometry/Geometry_Word_Problems.faq.question.262990.html

t=10
s=side of square
t/s=2.155
2.155s=t
s=t/2.155
s-10/2.155=4.64

4.64^2=21.53
also 
http://gmatclub.com/forum/an-equilateral-triangle-has-a-square-inscribed-in-it-side-139965.html