The ratio is 0.5mm: 4cm so you write the scale drawing's proportion as 4.5mm: __cm. To solve this, you make them fractions: 0.5/4 = 4.5/x. It helps if you write this out with the lines instead of the slashes :). So you cross multiply: 4 X 4.5= 18, then you do 18 divided by 0.5 to get an answer of 36. The actual height is 36cm