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Julie drives 100 mi to Grandmother's house. On the way to Grandmother's, Julie drives half the distance at 40.0 mph and half the distance at 60.0 mph . On her return trip, she drives half the time at 40.0 mph and half the time at 60.0 mph.

Required:
a. What is Julie's average speed on the way to grandmother's house?
b. What is her average speed in the return trip?


Sagot :

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!