# Finding Average Speed

You jog at 9.5 km/h for 7.0 km , then you jump into a car and drive an additional 18 km/h. With what average speed must you drive your car if your average speed for the entire 25 km is to be 22 km/h ?

 ebaines Posts: 10,594, Reputation: 5809 Expert #2 Sep 3, 2009, 02:52 PM

I think you mean to say that the car is driven for 18 km (distance), not 18 km/hr (speed), right?

The way to attack this kind of problem is to set up an equation for speed in terms of km per hour. The total distance traveld is 25 km. In order to have an average speed of 22 km/hr what is the total time the trip must take? You find that from:

$
\frac {25 km} {T hr} = 22 Km/hr.
$

Solve for T.

The total time is also equal to the sum of the time jogging plus the time driving, and each of these can be foun by dividing the distance tracveled by the speed. So:

$
T = \frac {7 km} {9.5 Km/hr} + \frac {18 km} {X km/hr}
$

Now you can solve for X.

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