Let $k$ be  a real  number. Define on the set of reals the operation $x*y$= $\frac{xy}{x+y+k}$ whenever $x+y$ does not equal $-k$. Let $x_1<x_2<x_3<x_4$ be the roots of $t^4=27(t^2+t+1)$.suppose that $[(x_1*x_2)*x_3]*x_4=1$. Find all possible values of $k$
1. Given the polynomial equation \( t^4 = 27(t^2 + t + 1) \), we can rewrite it as:
   \[
   t^4 - 27t^2 - 27t - 27 = 0
   \]
   Let \( x_1, x_2, x_3, x_4 \) be the roots of this polynomial.

2. By Vieta's formulas, we have the following relationships for the roots:
   \[
   x_1 + x_2 + x_3 + x_4 = 0 \quad \text{(sum of the roots)}
   \]
   \[
   x_1x_2 + x_1x_3 + x_1x_4 + x_2x_3 + x_2x_4 + x_3x_4 = -27 \quad \text{(sum of the product of the roots taken two at a time)}
   \]
   \[
   x_1x_2x_3 + x_1x_2x_4 + x_1x_3x_4 + x_2x_3x_4 = 27 \quad \text{(sum of the product of the roots taken three at a time)}
   \]
   \[
   x_1x_2x_3x_4 = -27 \quad \text{(product of the roots)}
   \]

3. We need to compute \( [(x_1 * x_2) * x_3] * x_4 \) using the operation \( x * y = \frac{xy}{x + y + k} \).

4. First, compute \( x_1 * x_2 \):
   \[
   x_1 * x_2 = \frac{x_1 x_2}{x_1 + x_2 + k}
   \]

5. Next, compute \( (x_1 * x_2) * x_3 \):
   \[
   (x_1 * x_2) * x_3 = \frac{\left( \frac{x_1 x_2}{x_1 + x_2 + k} \right) x_3}{\frac{x_1 x_2}{x_1 + x_2 + k} + x_3 + k} = \frac{x_1 x_2 x_3}{x_3 (x_1 + x_2 + k) + x_1 x_2 + k (x_1 + x_2 + k)}
   \]

6. Finally, compute \( [(x_1 * x_2) * x_3] * x_4 \):
   \[
   [(x_1 * x_2) * x_3] * x_4 = \frac{\left( \frac{x_1 x_2 x_3}{x_3 (x_1 + x_2 + k) + x_1 x_2 + k (x_1 + x_2 + k)} \right) x_4}{\frac{x_1 x_2 x_3}{x_3 (x_1 + x_2 + k) + x_1 x_2 + k (x_1 + x_2 + k)} + x_4 + k}
   \]

7. Given that \( [(x_1 * x_2) * x_3] * x_4 = 1 \), we equate the expression to 1 and solve for \( k \):
   \[
   \frac{\left( \frac{x_1 x_2 x_3}{x_3 (x_1 + x_2 + k) + x_1 x_2 + k (x_1 + x_2 + k)} \right) x_4}{\frac{x_1 x_2 x_3}{x_3 (x_1 + x_2 + k) + x_1 x_2 + k (x_1 + x_2 + k)} + x_4 + k} = 1
   \]

8. Simplifying the equation, we get:
   \[
   x_1 x_2 x_3 x_4 = x_1 x_2 x_3 + x_1 x_2 x_4 + x_1 x_3 x_4 + x_2 x_3 x_4 + k (x_1 x_2 + x_1 x_3 + x_1 x_4 + x_2 x_3 + x_2 x_4 + x_3 x_4) + k^2 (x_1 + x_2 + x_3 + x_4) + k^3
   \]

9. Substituting the values from Vieta's formulas:
   \[
   -27 = 27 + k(-27) + k^3
   \]

10. Solving the equation:
    \[
    -27 = 27 - 27k + k^3
    \]
    \[
    k^3 - 27k + 54 = 0
    \]

11. Factoring the cubic equation:
    \[
    (k - 3)(k^2 + 3k - 18) = 0
    \]

12. Solving for \( k \):
    \[
    k = 3 \quad \text{or} \quad k^2 + 3k - 18 = 0
    \]
    \[
    k = 3 \quad \text{or} \quad k = \frac{-3 \pm \sqrt{9 + 72}}{2} = \frac{-3 \pm 9}{2}
    \]
    \[
    k = 3 \quad \text{or} \quad k = 3 \quad \text{or} \quad k = -6
    \]

13. Therefore, the possible values of \( k \) are:
    \[
    k = 3 \quad \text{or} \quad k = -6
    \]

The final answer is \( \boxed{ k = 3 } \) or \( k = -6 \)