10 mm = 1 cm 100 cm = 1 m 1000 m = 1 km
1000 g = 1 kg 1000 kg = 1 tonne
10 ml = 1 cl 100 cl = 1 litre 1000 ml = 1 litre

Changing to a smaller unit gives a bigger number, so multiply: 3.4 km = 3.4 × 1000 = 3400 m. Changing to a bigger unit gives a smaller number, so divide: 650 g = 650 ÷ 1000 = 0.65 kg.

Time is not decimal. 0.4 of an hour is 0.4 × 60 = 24 minutes, so 2.4 hours is 2 hours 24 minutes, not 2 hours 40 minutes. Going the other way, 135 minutes = 135 ÷ 60 = 2.25 hours.

A square 1 m by 1 m is also 100 cm by 100 cm, so

1 m² = 100 × 100 = 10000 cm²
1 m³ = 100 × 100 × 100 = 1000000 cm³
1 litre = 1000 cm³

So 3.5 m² is 35000 cm², not 350 cm². For volume, a cuboid tank 50 cm by 40 cm by 30 cm holds 60000 cm³, which is 60000 ÷ 1000 = 60 litres, or 0.06 m³.

A scale of 1 : 50000 means 1 cm on the map is 50000 cm in real life. Both numbers are in the same unit.

1. 4.2 cm on a 1 : 50000 map
2. real distance = 4.2 × 50000 = 210000 cm
3. 210000 cm ÷ 100 = 2100 m
4. 2100 m ÷ 1000 = 2.1 km

To go from real life to the map, change the real distance to centimetres first, then divide by the scale number. On a scale drawing of a room at 1 : 50, a 9 cm line is 9 × 50 = 450 cm = 4.5 m.