The lengths of corresponding sides of 2 proportional rectangles are 20cm. and 8cm. the width of the larger rectangle is 5cm. what is the proportion used to find the width of the smaller rectangle?

In this case, we have 2 proportional rectangles. That means the ratio of their width:length  will be same. The bigger rectangle length 20 cm and width is 5cm. Then to find the smaller rectangle width, you need to multiply the smaller rectangle length with width:length ratio. The ratio would be:

width:length ratio= bigger rectangle width/ bigger rectangle length= 5cm/20cm= 1/4

Then the smaller rectangle width is 8cm * 1/4= 2cm