In the xy-plane, a triangle has vertices (0,0), (4,0) and (4,10). If a point (a,b) is selected at random from the triangular region, What is the probability that a-b>0? Choose the most appropriate option.
A) 1 / 5  B) 1 / 3  C) 1 / 2  D) 2 / 3  E) 2 / 5

Rationale: the area of the right triangle is ( 1 / 2 ) * 4 * 10 = 20 . only the points ( a , b ) below the line y = x satisfy a - b > 0 . the part of the triangle which is below the line y = x has an area of ( 1 / 2 ) ( 4 ) ( 4 ) = 8 . p ( a - b > 0 ) = 8 / 20 = 2 / 5 the answer is e .
Answer: E