Calculate Average Speed: Multi-Segment Journey with 78 km/h and 85 km/h

Q: Why do I include the coffee break time if he wasn't driving?

Average speed measures the entire journey from start to finish! Even though he stopped for coffee, that time is part of his total travel time. Think of it as door-to-door speed.

Q: Can I just average the two speeds (78 + 85) ÷ 2?

No! This only works if you drive for equal time periods at each speed. Since he drove 2 hours at 78 km/h but only 1.5 hours at 85 km/h, you must use the distance formula.

Q: What if there's an error in the calculation like 285 ÷ 3.75?

The explanation shows a small typo: it should be $ \frac{283.5}{3.75} = 75.6 $ km/h. Always double-check your distance calculation before dividing!

Q: How do I convert 15 minutes to hours?

Divide minutes by 60: $ \frac{15}{60} = 0.25 $ hours. Remember that 1 hour = 60 minutes, so any minute value divided by 60 gives you the decimal hour equivalent.

Average Speed with Travel Breaks

A man drives for two hours at a speed of 78 km/h, stops to get a coffee for fifteen minutes, and then continues for another hour and a half at a speed of 85 km/h.

What is his average speed?

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Understand the problem

A man drives for two hours at a speed of 78 km/h, stops to get a coffee for fifteen minutes, and then continues for another hour and a half at a speed of 85 km/h.

What is his average speed?

Step-by-step solution

In the first stage, we want to find the distance the truck traveled in its total journey,

We will use the data we already have,

78 km/h for two hours of driving and 85 km/h for an additional hour and a half.

78*2+85*1.5=

156+127.5=

283.5 km

Now, we want to discover the total duration of the journey.

We know there were two hours of driving, a quarter-hour break, and another hour and a half of driving,

Meaning:

2+0.25+1.5=

3.75 hours

Now, we'll divide the travel distance by the number of hours

285/3.75=

75.6 km/h

And that's the average speed!

Final Answer

$75.6$ km/h

Key Points to Remember

Essential concepts to master this topic

Formula: Average speed equals total distance divided by total time
Technique: Calculate: 78×2 + 85×1.5 = 283.5 km distance
Check: Include break time: 2 + 0.25 + 1.5 = 3.75 hours total ✓

Common Mistakes

Avoid these frequent errors

Forgetting to include break time in total duration
Don't calculate using only driving time (3.5 hours) = 81 km/h! This ignores stops and gives an inflated speed. Always include all time spent during the journey, including breaks and stops.

Practice Quiz

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What is the average speed according to the data?

FAQ

Everything you need to know about this question

Why do I include the coffee break time if he wasn't driving?

Average speed measures the entire journey from start to finish! Even though he stopped for coffee, that time is part of his total travel time. Think of it as door-to-door speed.

Can I just average the two speeds (78 + 85) ÷ 2?

No! This only works if you drive for equal time periods at each speed. Since he drove 2 hours at 78 km/h but only 1.5 hours at 85 km/h, you must use the distance formula.

What if there's an error in the calculation like 285 ÷ 3.75?

The explanation shows a small typo: it should be $\frac{283.5}{3.75} = 75.6$ km/h. Always double-check your distance calculation before dividing!

How do I convert 15 minutes to hours?

Divide minutes by 60: $\frac{15}{60} = 0.25$ hours. Remember that 1 hour = 60 minutes, so any minute value divided by 60 gives you the decimal hour equivalent.

What if the journey has multiple stops?

Use the same method! Add up all distances for each driving segment, then add up all time including every stop. The formula Average Speed = Total Distance ÷ Total Time always works.

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