If the average (arithmetic mean) of a and b is 45 and the average of b and c is 80, what is the value of c âˆ’ a?  Select the correct option.
A) 25 B) 70 C) 90 D) 140 E) it can not be determined from the information given

Rationale: "the arithmetic mean of a and b = ( a + b ) / 2 = 45 - - a + b = 90 - - 1 similarly for b + c = 160 - - 2 subtracting 1 from 2 we have c - a = 70 ; answer : b"
Answer: B