Australian AMC 2015 - Intermediate Question 10

Question 10

The diagram shows two small squares in opposite corners of a large square. The squares have sides of length 1 cm, 2 cm and 7 cm.

What is the area of the shaded pentagon?

Question illustration

$18\text{ cm}^2$
$16\text{ cm}^2$
$22\text{ cm}^2$
$24\text{ cm}^2$
$20\text{ cm}^2$