Arithmetic Grade Six

Lesson One Hundred Sixty-eight: Time and Distance


Sometimes an object does not travel at a constant speed. Consider the following problem: 


	  Jill's mom drove for 2 hours at a speed of 40 mph and
	then she drove 2 more hours at a speed of 50 mph. What
	was her average speed on this trip?
Two Step Solution:
  1. Find total distance (2 hours * 40 mph + 2 hours * 50 mph = 180 miles)
  2. Find average speed by dividing total distance by total time (180 miles ÷ 4 hours = 45 mph)

Here's another example: 


	  Doug's dad drove for 3 hours at a speed of 25 mph and
	then he drove 2 more hours at a speed of 20 mph. What
	was his average speed on this trip?
Two Step Solution:
  1. Total distance: 3 hrs * 25 mph + 2 hrs * 20 mph = 115 miles
  2. Average Speed: 115 miles ÷ 5 = 23 mph
Here are some examples showing how to find speed from distance and time: 

	DISTANCE  ÷  TIME  =  SPEED
	 40 miles ÷  2 hrs =  20 mph
	 75 miles ÷  3 hrs =  25 mph
	100 miles ÷  2 hrs =  50 mph
	120 miles ÷  4 hrs =  30 mph

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