A police cruiser, approaching a right-angled intersection from the north, is chasing a speeding car that has turned the corner and is now moving straight east. When the cruiser is $0.6$ km north of the intersection and the car is $0.8$ km to the east, the police determine with radar that the distance of the car is increasing at $20$ km/h. Suppose that the cruiser is moving at $60$ km/h at the instant of measurement. $\displaystyle \frac{d^{2}s}{dt^{2}}$ equal to at the moment in question if $\displaystyle \frac{d^{2}x}{dt^{2}}=-40$ and $\displaystyle \frac{d^{2}y}{dt^{2}}=50$(in km/h$^{2}$)