Let length of the longer diagonal = d cm
Then, length of other diagonal = 0.8 x d cm
Area of rhombus = (1/2) x d x 0.8 x d = 2/5 d2
= 2/5 d2
Area of square of the length of the longer diagonal = d2
So the area of the rhombus is 2/5 times the square of the length of the longer diagonal.