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Answer:
a. The average speed on her way to Grandmother's house is 48.08 mph
b. The average speed in the return trip is 50 mph.
Explanation:
The average speed (S) can be calculated as follows:
[tex] S = \frac{D}{T} [/tex]
Where:
D: is the total distance
T: is the total time
a. To find the total distance in her way to Grandmother's house, we need to find the total time:
[tex]T_{i} = t_{1_{i}} + t_{2_{i}} = \frac{d_{1_{i}}}{v_{1_{i}}} + \frac{d_{2_{i}}}{v_{2_{i}}}[/tex]
Where v is for velocity
[tex] T = \frac{d_{1_{i}}}{v_{1_{i}}} + \frac{d_{2_{i}}}{v_{2_{i}}} = \frac{(100/2) mi}{40.0 mph} + \frac{(100/2) mi}{60.0 mph} = 1.25 h + 0.83 h = 2.08 [/tex]
Hence, the average speed on her way to Grandmother's house is:
[tex]S_{i} = \frac{D}{T_{i}} = \frac{100 mi}{2.08 h} = 48.08 mph[/tex]
b. Now, to calculate the average speed of the return trip we need to calculate the total time:
[tex]D = v_{1_{f}}\frac{T_{f}}{2} + v_{2_{f}}\frac{T_{f}}{2} = \frac{T_{f}}{2}(v_{1_{f}} + v_{2_{f}})[/tex]
[tex]100 mi = \frac{T_{f}}{2}(40 mph + 60 mph)[/tex]
[tex] T_{f} = \frac{200 mi}{40 mph + 60 mph} = 2 h [/tex]
Therefore, the average speed of the return trip is:
[tex]S_{f} = \frac{D}{T_{f}} = \frac{100 mi}{2 h} = 50 mph[/tex]
I hope it helps you!