The midpoints of the sides of a rectangle are joined by straight lines as shown - OCR - GCSE Maths - Question 13 - 2019 - Paper 1

The shaded region is formed by connecting the midpoints of the rectangle. The area of this shaded region is half of the area of the rectangle, hence:

$Area_{shaded} = \frac{1}{2} \times A = \frac{1}{2} \times 4ab = 2ab$

To find the percentage of the rectangle that is shaded, use the formula:

$Percentage_{shaded} = \left( \frac{Area_{shaded}}{Area} \times 100 \right) = \left( \frac{2ab}{4ab} \times 100 \right) = 50\%$