unit 2 sec 1.4

To find the time required for each part of the journey, you needed to use values for the average speeds on each type of road. The average speed of the car on a road depends on many factors, such as how busy the road is, this speed limits, and if there are any delays such as gold works.

For current purpose, you should use the following cautions assumptions about average speeds on these types of roads:

v Average speed principal roads: 50km/h

v Average speed on motorways: 100km/h

Strategy

First method

Find the time to travel 1km and then find the time to travel the whole distance.

Second method

Time =

The unit of speed used in this formula is the one that involves the distance unit and time unit.

Example 4 calculating the time on the motorway.

Find the time for a journey of 30 km at an average speed 100km/h. give your answer in minutes.

Solution

First method

The average speed is 100km/h, so 1km is travelled in  h

Hence to travel 30km, it takes 30 ×  = 0.3h. Since there are 60 minutes in 1 hour

0.3h = (0.3 × 60) min = 18 min

Second method

By the formula, the time is

h = 0.3h

Multiplying by 60 to convert the time into minutes gives 18 min, as before.

This possible here because the numbers in the calculation are easy to work with. You can obtain a rough estimate of the answer by rounding the numbers in the calculation and then using an informal method of this type.