The first ten prime no's are :

2, 3, 5, 7, 11, 13, 17, 23, 29, 31

Clearly, the data is in ascending order. and

No of terms, n = 10

We know that, if even no of terms or observations are given, then the median of data is mean of the values of term and term. Where n is no of terms.

In this case, n = 10

i.e. median of above data is mean of 5^th and 6^th term

⇒ median

⇒ median = 12