Australian AMC 2018 - Upper Primary Question 21

Question 21

A rectangle measures 3 cm by 4 cm. A diagonal stripe is shaded which starts 1 cm from the diagonal corners, as in the diagram.

What fraction of the area of the rectangle is this shaded strip?

Question illustration

$\frac12$
$\frac13$
$\frac14$
$\frac15$
$\frac25$