The average of 1st 3 of 4 numbers is 6 and of the last 3 are 5. If the sum of the first and the last number is 11. What is the last numbers?
Choose one: A) 4 B) 5 C) 6 D) 7 E) 8

Rationale: "a + b + c = 18 b + c + d = 15 a + d = 11 a – d = 3 a + d = 11 2 d = 8 d = 4 answer : a"
Answer: A