Kali left school and traveled toward her
friend's house at an average speed of 40
km/h. Matt left one hour later and traveled
in the opposite direction with an average
speed of 50 km/h. Find the number of hours
Matt needs to travel before they are 400 km
apart.

Question

Kali left school and traveled toward her
friend's house at an average speed of 40
km/h.  Matt left one hour later and traveled
in the opposite direction with an average
speed of 50 km/h.  Find the number of hours
Matt needs to travel before they are 400 km
apart.

anonymous · Answer

At the time of Matt's departure, Kali is already 40km away.

From then on the rate of change will be ( 40km + 50km ) per hour.
There the equation would be  40 + 90n, where n is the number of hours passed.

If 40 + 90n = 400
Rearrange the equation to find n.

400-40 = 90n
n = 360/90
n = 4 hours