Suppose that the number of terms in an A.P. is \( 2k , \; k \in \mathbb{N} \). If the sum of all odd terms of the A.P. is \( 40 \), the sum of all even terms is \( 55 \) and the last term of the A.P. exceeds the first term by \( 27 \), then \( k \) is equal to:
