10. If on a building plan a line representing a wall is 90 mm and the plan has a scale of \( 1: 100 \), how many metres long is the actual wall?

To calculate the actual length of the wall, you first need to understand the scale. A scale of \( 1: 100 \) means that 1 mm on the plan equals 100 mm in reality. So, if the plan shows the wall as 90 mm, you multiply that by 100. Thus, \( 90 \, \text{mm} \times 100 = 9000 \, \text{mm} \). Since there are 1000 mm in a metre, you divide by 1000 to convert millimetres to metres, which gives you 9 metres. So, the actual length of the wall is 9 metres!