Explanation:

the given pattern is 3,15, 8, 24, 15, 35, ? , 48, 35
let the missing term be y
here we can see that 2 series are running
1st series : 3 , 8 , 15 , y , 35
2nd series : 15, 24 , 35 , 48
as the missing term is a part of 1st pattern then
8 – 3 = 5
15 – 8 = 7
as the difference is increasing by 2 every time and hence
y – 15 = 9
y = 24