$1, 2, 4, 6, 8, 9$ transformed into another
series whose mean is $12$. Two middle terms of the transformed
series are

Mean of the given series is $5.$ If we have to
transfer the given series into one having $12$as mean, we
must take$d = x + 7.$ The transformed terms are $8, 9, 11, 13, 15, 16.$ The two middle terms are $11, 13.$