Question

What’s Ali’s average speed on her walk to school if she walks halfway at 2mph and runs the rest of the way at 7mph?

1. thongdat2
s = 28/9
Step-by-step explanation:
d/(7 x 2) + d/(2 x 2) = d/s
d/14 + d/4 = d/s
18d/56 = d/s
18/56 = 1/s
s = 28/9

2. mocmien2
The average speed is 28/9.

### What is Average speed?

Average speed is the mean value of the speed of a body over a period of time. The formula for average speed is needed since the speed of a moving body is not constant and varies across a period of time. Even with varying speed, the values of total time and the total distance covered can be used, and with the help of the formula for average speed, we can find a single value to represent the entire motion.
Example
Using the average speed formula, find the average speed of Sam, who covers the first 200 kilometers in 4 hours and the next 160 kilometers in another 4 hours.
To find the average speed we need the total distance and the total time.
Total distance covered by Sam = 200Km + 160 km = 360 km
Total time taken by Sam = 4 hour + 4 hour = 8 hour
Average Speed = Total distance covered ÷ Total time taken
Average Speed = 360 ÷ 8 = 45km/hr
The average speed of a body is equal to the total distance covered, divided by the total time taken. The formula for average speed is given as:
Average Speed Formula:
Average Speed = Total distance covered ÷ Total time taken
Given:
halfway at 2mph and runs the rest of the way at 7mph
d/(7 x 2) + d/(2 x 2) = d/s
d/14 + d/4 = d/s
18d/56 = d/s
18/56 = 1/s
s = 28/9 mph