Description:
Distance, Rate and Time
(or DRT)
questions ask you to solve for any of the
three variables in the DRT equation:
Approach:
In order to solve the more difficult questions from this category, you
must resist the temptation to average different speeds. Average speed is always
total distance divided by total time, and not the average of the speeds.
1) Average Speed is NOT the Average of Your Speeds
In order to properly calculate an average speed for a series of different
trips, first determine the total time and the total distance for the trips combined. Divide
total distance by total time to determine average speed.
Average Speed = Total Distance / Total Time.
2) Practice:
Tatum averaged 30 mph on her drive to work. She
averaged 60 mph on her drive home. What was Tatum's average speed for
the round trip?
-
First, make up a distance like 120 miles (which is a multiple of both 30 and
60).
-
Second, determine the time for the 30 mph trip by dividing.
120 / 30 = 4
-
Third, determine the time for the 60 mph trip by dividing.
120 / 60 = 2
-
Finally, divide the total distance (120 + 120)
by the total time (4 + 2) to
determine the average speed.
240 / 6 = 40
3) Practice:
If Susan drives to school at 40 mph and returns home
at 60 mph, what was her average speed for the round trip?
-
44
-
48
-
50
-
56
-
58
