How to find the value of x given the following rectangular that has been divided into 4 parts?

Question

A rectangle is divided into sections with the area shown. What is the value of X?

At first I didn't know what i would do than i noticed that X is 1/4th of the rectangular . But I don't know what to do next with this.

Answer 1

Let $a,b,c,d$ be the sides of the little rectangles (as shown in the picture).

$ab=24$, $ac=40$, $bd=21$, so $x=cd=\frac{(ac)(bd)}{ab}=\frac{40\cdot 21}{24}=35$.