Speed, Distance, Time

Example: Timothy walks to school which is $2.5$ kilometres away from his house.

He walks at an average speed of $5$ kilometres per hour.

How many minutes does it take Timothy to walk to school?

The version of the formula that is most useful for us in this question is:

Time $=$ Distance $div$ Speed

The distance is $2.5$ kilometres and the speed is $5$ kilometres per hour.

Your child then needs to be able to substitute these values into the formula.

Time $= 2.5 div 5 = 0.5$ hours 

The question wants our answer in minutes.

There are $60$ minutes in an hour.

$0.5 times 60 = 30$ minutes

So, it will take Timothy $30$ minutes to walk to school.

Example: Simraj takes the bus to visit her friend’s house.

The bus takes $2$ hours and $45$ minutes to get to her friend’s house and travels at an average speed of $20$ miles per hour.

First, your child needs to convert the time into hours as a decimal.

$45$ minutes is $dfrac{3}{4}$ of an hour or $0.75$ of an hour.

So, the time becomes $2.75$ hours.

Next, your child can choose the appropriate formula to use.

Distance $=$ Speed $times$ Time

The speed is $20$ miles per hour and the time is $2.75$ hours.

Substitute the values into the correct version of the formula.

Distance $=20 times 2.75 = 55$ miles

So, the distance between Simraj’s house and Simraj’s friend’s house is $55$ miles.

Example: Sian has taken her dog for a walk in the park.

She times how long it takes for her dog to run $800$ metres.

It takes her dog $3$ minutes to run that distance.

Work out the average speed of Sian’s dog.

Give your answer in kilometres per hour.

As the question wants our answer in kilometres per hour, we need to convert the distance to kilometres and the time to hours.

There are $1000$ metres in $1$ kilometre.

$800 div 1000 = 0.8$ kilometres

There are $60$ minutes in $1$ hour.

$3 div 60 = 0.05$ hours

Now that the values are in the correct form we can calculate the average speed using the formula:

Speed $=$ Distance $div$ Time

Speed $= 0.8 div 0.05 = 16$ kilometres per hour

So, the average speed of Sian’s dog is $16$ kilometres per hour.