In 3 metres there are 300 centimetres. Each centimetre on the scale is 300 centimeters, or a ratio of 1:300. The tree is 5 centimetres on the scale. Each centimeter on the scale is 300 centimetres, therefore 5 centimetres on the scale is 1500 centimetres. 1500 centimetres is equal to 15 metres.