Let $\triangle ABC$ be a triangle with $AB=6, BC=8, AC=10$, and let $D$ be a point such that if $I_A, I_B, I_C, I_D$ are the incenters of the triangles $BCD,$ $ ACD,$ $ ABD,$ $ ABC$, respectively, the lines $AI_A, BI_B, CI_C, DI_D$ are concurrent. If the volume of tetrahedron $ABCD$ is $\frac{15\sqrt{39}}{2}$, then the sum of the distances from $D$ to $A,B,C$ can be expressed in the form $\frac{a}{b}$ for some positive relatively prime integers $a,b$. Find $a+b$.

[i]Proposed by [b]FedeX333X[/b][/i]
1. **Given Data and Setup:**
   - We have a triangle $\triangle ABC$ with sides $AB = 6$, $BC = 8$, and $AC = 10$.
   - Point $D$ is such that the lines $AI_A, BI_B, CI_C, DI_D$ are concurrent, where $I_A, I_B, I_C, I_D$ are the incenters of triangles $BCD, ACD, ABD, ABC$ respectively.
   - The volume of tetrahedron $ABCD$ is given as $\frac{15\sqrt{39}}{2}$.

2. **Concurrent Lines and Angle Bisector Theorem:**
   - For the lines $AI_A, BI_B, CI_C, DI_D$ to be concurrent, we use the property that the angle bisectors of a triangle are concurrent at the incenter.
   - By the converse of the angle bisector theorem, if $DI_B$ and $BI_D$ meet at the same point on $AC$, then $DC \cdot AB = DA \cdot BC$.

3. **Unfolding the Tetrahedron:**
   - Consider the faces $DAC$ and $BAC$. Unfolding along $AC$ gives quadrilateral $DABC$.
   - Since $DI_B$ and $BI_D$ are angle bisectors, we apply the converse of the angle bisector theorem to get $DC \cdot AB = DA \cdot BC$.

4. **Volume Calculation and Distance Relations:**
   - Given the volume of tetrahedron $ABCD$ is $\frac{15\sqrt{39}}{2}$, we can use the formula for the volume of a tetrahedron:
     \[
     V = \frac{1}{6} \left| \vec{AB} \cdot (\vec{AC} \times \vec{AD}) \right|
     \]
   - Using the given volume, we can find the height from $D$ to the base $\triangle ABC$.

5. **Finding Distances $DA, DB, DC$:**
   - Let $DA = 15k$, $DB = 12k$, and $DC = 20k$ for some positive real $k$.
   - Using the volume formula and the given volume, we solve for $k$:
     \[
     \frac{1}{6} \times \text{Area of } \triangle ABC \times \text{height} = \frac{15\sqrt{39}}{2}
     \]
   - The area of $\triangle ABC$ can be found using Heron's formula:
     \[
     s = \frac{6 + 8 + 10}{2} = 12
     \]
     \[
     \text{Area} = \sqrt{12(12-6)(12-8)(12-10)} = \sqrt{12 \times 6 \times 4 \times 2} = \sqrt{576} = 24
     \]
   - The height from $D$ to $\triangle ABC$ is:
     \[
     \text{height} = \frac{6 \times \frac{15\sqrt{39}}{2}}{24} = \frac{15\sqrt{39}}{8}
     \]

6. **Sum of Distances:**
   - The sum of the distances from $D$ to $A, B, C$ is:
     \[
     DA + DB + DC = 15k + 12k + 20k = 47k
     \]
   - Solving for $k$ using the height:
     \[
     k = \frac{1}{2}
     \]
   - Therefore, the sum of the distances is:
     \[
     47k = 47 \times \frac{1}{2} = \frac{47}{2}
     \]

The final answer is $\boxed{49}$.