Question 259071
A square's side is increased by 10 yards, which corresponds to an increase in the area by 400 square yards. 
How many yards is the side of the initial square?
:
Let x = side of the original square
then
(x+10) = side of the larger square
:
Large area - small area = 400 sq/yd
(x+10)^2 - x^2 = 400
FOIL
x^2 + 20x + 100 - x^2 = 400
20x = 400 - 100
20x = 300
x = {{{300/20}}}
x = 15 yds is the original square
;
:
Check it:
25^2 - 15^2 = 
625 - 225 = 400