$\eqalign{  & \frac{{120a + (16 \times 240)}}{{42a + (16 \times 137/5)}} \times 100 = 240 \Rightarrow \frac{{120a + 3840}}{{42a + 2200}} \times 10 = 24  \cr   &  \Rightarrow 1200a + 38400 = 1008a + 52800  \cr   &  \Rightarrow (1200 - 1008)a = 52800 - 38400  \cr   &  \Rightarrow a = \frac{{52800 - 38400}}{{1200 - 1008}} = \frac{{14400}}{{192}} = 75 \cr} $