The diagonalof a rectangle is 25 inches. the width is 15 inches. what is the area of the rectangle?

Hey there! I'm not sure what method your teacher would be looking for but my first thought would be using the Pythagorean theorem! 

a^2 + b^2 = c^2
Where a and b are the legs while c is the hypotenuse

In this case the diagonal would be the hypotenuse so you could plug that in for c and the width is one of the legs so you can plug that in too.
a^2 + 15^2 = 25^2

Then 
a^2 +225 = 625

Then 
a^2 = 400

Then 
a = 20

Now you know the width and length (15 and 20). So you can multiply both of them to get the area. So you answer is 300!!! 

I hope that helps! Feel free to let me know if you have any questions!

- mathwizzard3