hi i also have this other equation which i am unsure of, just wondering if you can help which is:

A square steel plate is pierced by a square tool leaving a margin of 20mm all round. The area of the hole is one third of that of the original plate. calculate the dimensions of the original plate.

thanks again!
jenko!

2. Hello, jenko!

Did you make a sketch?

A square steel plate is pierced by a square tool leaving a margin of 20mm all round.
The area of the hole is one-third of that of the original plate.
Calculate the dimensions of the original plate.
Code:
      :  20 :   x-40    :  20 :
-  * - - - - - - - - - - - * -
20  |                       | :
-  |     * - - - - - *     | :
:  |     |           |     | :
:  |     |           |     | :
x-40 |     | x-40      |     | x
:  |     |           |     | :
:  |     |    x-40   |     | :
-  |     * - - - - - *     | :
20  |                       | :
-  * - - - - - - - - - - - * -
: - - - - - x - - - - - :

The side of the original plate is: . $x$ mm.
. . Its area is: . $x^2$ mm².

The side of the hole is: . $x - 40$ mm.
. . Its area is: . $(x-40)^2$ m².

The area of the hole is one-third the area of the plate.
. . There is our equation! . . . . . $(x-40)^2 \:=\:\frac{1}{3}x^2$