The given AP is 6,13,20,........,216.
First term, a = 6
Common difference, d = 13 - 6 = 7
Suppose these are n terms in the given AP. Then,
an = 216
Thus, the given AP contains 31 terms,
∴ Middle term of the given AP
= \((\frac{31\,+\,1}{2})th\) term
= 16th term
= 6 + (16 - 1) x 7
= 6 + 105
= 111
Hence, the middle term of the given AP is 111.