# Thread: Working out speed from distance and time?

1. ## Working out speed from distance and time?

Hi all, I was wondering whether someone can enlighten me in the case of working out this easy question that has stumped me. I came across a question whereby I was required to work out the speed.

Question: A person in a car travel 10km in the space of 30minutes, what was their average speed?

I know how I would work this out using Speed = Distance / Time. However, on this occassion because I am doing a smaller number divided into a larger number it comes out as 0.333333 which is wrong. I know the answer it 20kmh but I don't know how to perform the working out. If someone can help me with this I would greatly appreciate it

2. Hi

It is a question of unit : if you want the speed expressed in km/h then the distance must be expressed in km and the time in h
Here : distance = 10 km and time = 30 minutes = 0.5h
Therefore speed = 10/0.5 = 20 km/h

3. Oh I see so must I always convert it into units and then perform the calculations? I found that if it for instance the problem was:

Distance = 30km
Time = 2 hours

Speed = 30/2 = 15kmh?

Would this be correct? Would I only convert them into units if the distance was of a greater number than the minutes.

4. $\displaystyle t = 30min = \frac{1}{2}h$
$\displaystyle s = 10km (= 10000m)$
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You can do it by changing everything into metres and seconds, OR kms (or m) and mins OR kms and hours OR something totally idiotic like nanometres and years (perfect for tests hehe)
The easiest and fastest way is usually the one in which the information is given (in this case hours and kms).

$\displaystyle \overline{v} = \frac {s}{t}$

$\displaystyle \overline{v} = \frac{10km} {\frac {1}{2}h} = \frac {10km \times 2}{1h} = 20 km/h$

5. Oh I see! I could multiply the time to make up the hour and apply the same to the distance thus giving me the speed.

Thank you kind person!