If Sajid’s scale is 1:32, the ratio of the length of his model and the lorry is, of course, 1/32. Meaning that the lorry is (32*45)cm long. So, it is 1440 cm. If Chitra’s model uses the scale 1:72, every 72 cm of length of the lorry is 1 cm on the model, meaning that the length of his model is (1440/72) cm= 20 cm