Vehicle speeds v1 = 54 km / h and v2 = 90 km / h, respectively. After what time interval will the distance between them be l = 12 km?

Most likely, the task involves the movement of cars in one direction. Then the speed of their removal: v2-v1 = 90-54 = 36 km / h. It turns out that in an hour, i.e. in 60 minutes the distance between them will be 36 km. And 12 km is a third of 36 km, we denote the required time for x, then 12/36 = x / 60; x = 12 • 60: 36, x = 20 min
Answer: 20 minutes