Ratio of expenditures of company A and B in 2012 was 3 : 4 respectively. What was the respective ratio of their income in 2012?

$$\eqalign{ & \text{For company A, In 2012} \cr & P\% = \frac{I-E}{E}\times100 \cr & \Rightarrow 30 = \frac{I-3}{3}\times100 \cr & \Rightarrow 90 = 100I-300\cr & \Rightarrow \frac{390}{100} = I \cr & \Rightarrow I = 3.9 \cr & \text{For company B, In 2012} \cr & P\% = \frac{I-E}{E}\times100 \cr & \Rightarrow 40 = \frac{I-4}{4}\times100 \cr & \Rightarrow 160 = 100I-400 \cr & \Rightarrow \frac{560}{100} = I \cr & \Rightarrow I = 5.6 \cr & \text{Then required ratio} \cr & = \frac{3.9}{5.6} \cr & = \frac{39}{56} \cr & = 39:56 \cr} $$