Let the A.P. be a,a+d,a+2d⋯⋯,l−3d,l−2d,l−d,l
Where a is the first term and l is the last term,
Sum of 1st, 3terms is, 39.
30+3d=39 as a=10 (Given)
d=39=3
Sum of last 4 terms is178.
4l−6d=178
l=49
10,13,16,19.......46,49
Let number of terms are, n then nthterm is10+(n−1)3=49
⇒n=14
So the median of the series would be mean of 7thand8th term
210+6⋅(3)+10+7⋅3
=259=29.5
Alternate way
The median would be mean of 10 and 49, That is 29.5.