The polynomial $x^3-kx^2+20x-15$ has $3$ roots, one of which is known to be $3$. Compute the greatest possible sum of the other two roots.

[i]2015 CCA Math Bonanza Lightning Round #2.4[/i]
1. **Identify the roots and use Vieta's formulas:**
   Given the polynomial \( P(x) = x^3 - kx^2 + 20x - 15 \) and one of the roots is \( 3 \), we can denote the roots as \( 3, x_1, x_2 \).

2. **Apply Vieta's formulas:**
   By Vieta's formulas for a cubic polynomial \( ax^3 + bx^2 + cx + d = 0 \), the sum of the roots taken one at a time is given by:
   \[
   r_1 + r_2 + r_3 = -\frac{b}{a}
   \]
   Here, \( a = 1 \) and \( b = -k \), so:
   \[
   3 + x_1 + x_2 = k
   \]
   Therefore:
   \[
   x_1 + x_2 = k - 3
   \]

3. **Sum of the product of the roots taken two at a time:**
   The sum of the product of the roots taken two at a time is given by:
   \[
   r_1r_2 + r_2r_3 + r_3r_1 = \frac{c}{a}
   \]
   Here, \( c = 20 \), so:
   \[
   3x_1 + 3x_2 + x_1x_2 = 20
   \]

4. **Product of the roots:**
   The product of the roots is given by:
   \[
   r_1r_2r_3 = -\frac{d}{a}
   \]
   Here, \( d = -15 \), so:
   \[
   3x_1x_2 = 15 \implies x_1x_2 = 5
   \]

5. **Substitute \( x_1x_2 = 5 \) into the sum of the product of the roots:**
   \[
   3x_1 + 3x_2 + 5 = 20 \implies 3(x_1 + x_2) + 5 = 20 \implies 3(x_1 + x_2) = 15 \implies x_1 + x_2 = 5
   \]

6. **Determine \( k \):**
   From \( x_1 + x_2 = k - 3 \):
   \[
   5 = k - 3 \implies k = 8
   \]

Thus, the greatest possible sum of the other two roots is \( 5 \).

The final answer is \(\boxed{5}\).