A rectangle is divided by two vertical and two horizontal segments into nine rectangular parts. The areas of some of the resulting parts are indicated in the figure. Find the area of the upper right part.

| 30 |  | $?$ |
| :---: | :---: | :---: |
| 21 | 35 |  |
|  | 10 | 8 |
It is not difficult to understand that if a rectangle is cut into 4 parts by a vertical and a horizontal segment, then the products of the areas of opposite parts are equal. Using this consideration, we sequentially find the areas of the upper middle part $(30 \cdot 35: 21=50)$, the right middle part $(8 \cdot 35: 10=28)$, and finally the upper right part $(50 \cdot 28: 35=40)$.

## Answer

40. 

## Problem