SOLUTION: the average speed of a car increases from 60km/h-70 km/h, the time needed to cover a certain distance decreases by 30 minutes. what is the distance involved?

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Question 213122: the average speed of a car increases from 60km/h-70 km/h, the time needed to cover a certain distance decreases by 30 minutes. what is the distance involved?
Answer by jonvaliente(64) About Me  (Show Source):
You can put this solution on YOUR website!
Since: Speed=Distance/Time
And Time = Distance/Speed
The distance did not change and we let this value be D
When Speed = 60, Time is T, as in:
...
T+=+D%2F60
...
When Speed = 70, Time is T-30 minutes(30 minutes is half an hour, speed is expressed as km/h) so, T-0.5, as in:
T-0.5+=+D%2F70+
...
Substituting the value of T from the first equation, we get:
%28D%2F60%29-0.5+=+D%2F70
%28D%2F60%29-%28D%2F70%29+=+0.5
Getting the LCD, 420 we get:
%287D-6D%29%2F420=0.5
Multiplying both sides by 420 we get:
D+=+0.5%2A420+=+210+km
So the distance involved is 210km