A scale of 1 : 50,000 means 1 cm on the map is 50,000 cm in real life, which is 500 m.

1. 4.2 cm on a 1 : 50,000 map
2. 4.2 × 50,000 = 210,000 cm
3. = 2,100 m = 2.1 km

Going the other way, divide: 3 km = 300,000 cm, and ÷ 50,000 gives 6 cm on the map.

Scale the lengths first, then work out the area. On a 1 : 100 plan, a 3 cm by 4 cm room is 3 m by 4 m, so 12 m². Multiplying 12 cm² by 100 would be wrong: areas scale by 100².