Given that m is a positive integer and 4^m - 1 = n, which of the following values CANNOT represent n

A. 3
B. 7
C. 63
D. 255

We know that:

• 4^1 = 4
• 4^2 = 16
  
• 4^3 = 64
  
• 4^4 = 256
  
• 4^5 = 1024
  
• 4^6 = 4096

Notice they all end in 4 or 6. This continues for to infinity.

4^m will either end in a 4 or a 6
Therefore, 4^m - 1 ends in:

• 4 - 1 = 3
• 6 - 1 = 5

Choices A, C, and D end in 3 or 5.

Choice B does not. So Choice B is the answer.