The curve represented by  $x=t^5+5t^3+20t+7$ and  $y=4t^3-3t^2-18t+3$ is decreasing in the interval   

$\begin{array}{rl} & \frac{dx}{dt}=5t^4+15t^2+20\\ & \frac{dy}{dt}=12t^2-6t-18\\ & \frac{dy}{dx}=\frac{\frac{dy}{dt}}{\frac{dx}{dt}}=\frac{6(2t^2-t-3)}{5(t^4+3t^2+4)}\end{array}$
Decreasing 
$\begin{array}{rl} & \Rightarrow \frac{dy}{dx}<0\Rightarrow \frac{6(2t^2-t-3)}{5(t^4+3t^2+4)}<0\\ & \Rightarrow 2t^2-t-3<0\Rightarrow (t+1)(2t-3)<0\\ & t\in (-1,3/2)\end{array}$