You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.
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Question: Question: Find the average of all prime numbers between 4 and 15.

 Option A: 30

 Option B: 4

 Option C: 9

 Option D: 8

 Option E: None

Answer: Option C


Question: Question: (3x + 1) (2x - 5) = ax^2 + kx + n . What is the value of a - n + k ?

 Option A: 5

 Option B: 8

 Option C: 9

 Option D: 14

 Option E: 11

Answer: Option D


Question: Question: In a friendship gang Andrew has 8 gang, in how many ways can he invite one or more of the gang to his house?

 Option A: 234

 Option B: 255

 Option C: 265

 Option D: 267

 Option E: 272

Answer:
Option B