If the average of $a,b,c$ is $15$, the average of $b,c,d$ is $12$ and the value of $a$ is $19$, the value of $d$ is:

Given information:
1. The average of $a,b,c$ is $15$:

$$\frac{a+b+c}{3}=15⟹a+b+c=45\text{— (Equation 1)}$$

2. The average of $b,c,d$ is $12$:

$$\frac{b+c+d}{3}=12⟹b+c+d=36\text{— (Equation 2)}$$

3. The value of $a$ is $19$. Substitute $a=19$ into Equation 1:

$$19+b+c=45⟹b+c=45-19=26$$

Now, substitute the value of $(b+c)=26$ into Equation 2:

$$26+d=36⟹d=36-26=10$$

Thus, the value of $d$ is $10$.