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
- $23$
- $17$
- $15$
- $14$