What Is the Area of the Yellow Rectangle?

A primary school math problem.

They are too many drama posts on the Internet, let’s wash our brains with this funny primary school math problem.

In the above picture, each number in the rectangle represents the area of this rectangle. So, what is the size of the yellow rectangle?

This is not a difficult question, but if you want to get the answer quickly, it requires a little trick.

The trick is to divide the yellow rectangle into two parts:

(To help discuss, I add serial numbers to each rectangle.)

Rectangle1 and reactangle2 share the width, but rectangle1 is 2 times bigger than rectangle2, so we can know that rectangle1 is 2 times longer than rectangle2.

So, we can deduce such a relationship:

42 + 9 + a = (18 + b) * 2

Also, since rectangle5 is 2 times bigger than rectangle4 while they have the same width, we can deduce that rectangle5 is 2 times longer than rectangle4.

So, we can deduce such a relationship:

b = a * 2

Two equations for 2 unknowns, it’s very simple: a is 5, b is 10.

So the area of the yellow rectangle is 15.

Here is another solution that does not require any algebra! Only ratios and subtraction!

By Lew Baxter: