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| Math Central | Quandaries & Queries |
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Question from Natalie, a student: Mr Lim drove at 70 km per hour for 1 hour and 30 minutes. He then drove another 55 km. if his average speed for the whole journey was 80km per hour, what was his speed for the 55 km journey? |
Hi Natalie,
The key to this problem is that
distance = time times rate.
You need to use this relationship three times, once for the first part of the journey, again for the second part of the journey and a third time for the complete journey.
For the first part the rate is 70 km per hour and the time is 1.5 hours so
distance = $1.5 \times 70$
and hence you can find the distance for the first part of the journey.
The distance for the second part is 55 km and the total distance is the sum of the distances for parts one and two and hence you can find the total distance.
Now look at the complete journey, you already knew the rate and now you know the distance so you can find the total time.
Time to look at the second part of the journey. Can you see how to complete the problem?
Penny
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