1. One stamp: The size of a standard stamp is about 2.5cm× 3.5cm, and the area is about 8.75cm The area that can accommodate three stamps is close to 30cm.

2. A credit card: The size of a standard credit card is 8.56cm× 5.4cm, with an area of about 46.224cm2, and 30cm is about 65% of the credit card area.

3. One page of the short book: the page size of the short book is about 10 cm× 15 cm, and the area is about 150 cm 2. 30 square centimeters is about 20% of a page.