m and n are the x and y coordinates, respectively, of a point in the coordinate plane. If the points (m, n) and (m + p, n + 15) both lie on the line defined by the equation x = (y/5) - (2/5), what is the value of p?  Choose the correct choice
A) 1
B) 2
C) 3
D) 4
E) 5

Rationale: "x = ( y / 5 ) - ( 2 / 5 ) , and so y = 5 x + 2 . the slope is 5 . ( n + 15 - n ) / ( m + p - m ) = 5 p = 3 the answer is c ."
Answer: C