In a soil investigation work at a site, Standard Penetration Test (SPT) was conducted at every $1.5\:m$ interval upto $30\: m$ depth. At $3\:m$ depth, the observed number of hammer blows for three successive $150\:mm$ penetrations were $8,6$ and $9$,respectively. The SPT N-value at $3\:m$ depth, is
1. $23$
2. $17$
3. $15$
4. $14$