Question

The wight of 72 books is 9 kg. What is the wight of such 40 books?

Answer 1

$$\begin{gathered} \mathrm{Weight}\mathrm{of}72\mathrm{books}=9\mathrm{kg} \\ \mathrm{Weight}\mathrm{of}1\mathrm{book}=\frac{9}{72}\mathrm{kg}=\frac{1}{8}\mathrm{kg} \\ \mathrm{Weight}\mathrm{of}40\mathrm{books}=\frac{1}{8}\times 40=5\mathrm{kg} \end{gathered}$$