Set the maximum speed limit to x km/h.
The car must stop when it leaves the obstacle 40-5=35 (meters).
In addition, within 1.5 seconds, the distance traveled by the car is:1.5×1/3600×××1000 = 5/12x (m).
(The interpretation of this formula is: 1.5 seconds is first passed by × 1/3600, and the unit is converted into "hours"; The distance is obtained by x, time x speed, but because the unit of x is km/h, the distance is obtained in km; Finally × 1000, which can be converted into "meters")
Therefore, 5/ 12x+ 1/60x2=35.
? x2+25x-35×60=0
(x+60)(x-35)=0
So you get x=35.