missing info

Length AB = Length CD

= ⅓ × length of rectangle
= 42 cm ÷ 3 = 14 cm (Or 42 cm × ⅓ = 14 cm)


Area of rightmost △ = ½ × base × height

= ½ × AB × breadth of rectangle

(Since the height and rectangle's breadth are the same length)

= ½ × 14 cm × 25 cm
= 175 cm²


Area of rectangle = length × breadth
= 42 cm × 25 cm
= 1050 cm²

Area of △EFC
= 1/5 the area of the rectangle
= 1/5 × 1050 cm² (or 1050 cm² ÷ 5)
= 210 cm²

Area of leftmost △

= ½ × base x height

= ½ × breadth of rectangle × CD

(Notice that the base is equal to breadth of the rectangle.

And if we look closely, the △height is level with point D. You can try drawing a dotted line from the top of the height to D. So CD is the same length as its height)

= ½ × 25 cm × 14 cm
= 175 cm²


Unshaded area

= Area of rectangle - areas of the three △s

= 1050 cm² - 175 cm² - 175 cm² - 210 cm²

= 490 cm²

Your mistakes were :

- You mixed up the calculation for ② with ③.

- In ③ (which the working was supposed to be for ②) , 14 × 25 is not correct as the length of rectangle is 42cm.

It should be 42 cm × 25 cm


- To find the unshaded area, you should be subtracting the shaded area from the area of rectangle.

But you wrote 420 - 390. That's subtracting 390 from shaded area.

And where is the 390 from?