How does the area of the triangle relate to the area of the square? ! i really don't get it!

A square has 4 right angles and 4 equal sides.
If you draw a diagonal through the square then you get two right triangles. So the area relation between a square and a right triangle is going to be half. The area of the triangle is half the area of the square. 

Also the formula for area of a square is length times width.
And the formula for the area of a right triangle is 0.5 times length times width.

So if you set up the equation (let's use * as multiplication symbol ): 

Area of a Square : Area of a Right Triangle
length * width : 0.5 * length * width
(Divide by length * width on both sides)
1:0.5 or 2:1

The correct answer is $90