Area of a triangle
Look at these right-angled triangles.
Move your mouse pointer over the triangles. You will see that each one is half a rectangle.
So the area of each triangle is half of its rectangle's area
This triangle is half of a rectangle which is 20cm wide and 12cm high.
area = 20cm x 12cm = 240cm2
So, the area of the triangle = 240 ÷ 2 = 120cm2