VTMO 2017 — Grade 8 — semifinal Question 10

Odd numbers are grouped: $\{ 1 \}$; $\{ 3 , 5 \}$; $\{ 7 , 9 , 11 \}$; $\{ 13 , 15 , 17 , 19 \}$; … What is the middle number of group $15$?