Clearly, 6,13,20,.....,216 is an A.P. with first term a=6 and common difference d=7.Let there be n terms in the given A.P. Then, an​=216⟹a+(n+1)d=216⟹6+7(n−1)=216          [∵a=6 and d=7]⟹7n=217⟹n=31Here, n is odd so (2n+1​)th i.e. (231+1​)th=16th term is the middle term and is given by a16​=a+(16−1)d=1+15d=6+15×7=111