In the figure above, what is the area of triangle QRS?

Answer:
D) 20.
Step-by-step explanation:
First find the value of x using Pythagoras theorem:
10^2 = x^2 + (2x)^2
5x^2 = 100
x^2 = 20
x = √20
= 2√5.
So the area of triangle
= 1/2 * base * height
= 1/2 * 2x * x
= x^2
= (2√5)^2
= 20.