Partial derivative problem

Em Yeu Anh

I have repeated this one several times, and keep getting the same answer. But it's marked wrong, can't find my error:

$$\displaystyle w = xe^{\frac{y}{z}}$$

$$\displaystyle x = t^4$$
$$\displaystyle y = 6 - t$$
$$\displaystyle z = 1 + 5t$$

$$\displaystyle \frac{dw}{dt} = e^{\frac{y}{z}}(4t^3) + xe^{\frac{y}{z}}(\frac{1}{z})(-1) + xe^{\frac{y}{z}}(\frac{-y}{z^2})(5)$$

$$\displaystyle \frac{dw}{dt} = e^{\frac{y}{z}}(4t^3-\frac{x}{z}-\frac{5xy}{z^2})$$