Another math problem... can someone help

I'm working on a practice SAT and can't figure out this problem...

'On a square gameboard that is divided into n rows of n squares each, k of these squares lie on the boundary of the gameboard. Which of the following is a possible value for k?

10
25
34
42
52'

Can someone please write an explanation for this?

equation is n*2 + (n-2)* 2

if n = 6, 6*2 + 4*2 = 20
if n = 7, 7*2 + 5*2 = 24
if n = 8, 8*2 + 6*2 = 28
if n = 10, 10*2 + 8*2 = 36
if n = 12, 12*2 + 10*2 = 44
if n = 14, 14*2 + 12*2 = 52

Answer is (E).

Each edge has n squares touching it, the total is 4n minus the duplicated at 4 corners. Therefore, the number should be

4n - 4

The only possible value is 52, which n = 14.

answer is here

http://www.algebra.com/algebra/homework/word/misc/Miscellaneous_Word_Problems.faq.question.393640.html