A, B and C enter into a partnership in the ratio 72 :43 : 65 . After 4 months, A increases his share 50%. If the total profit at the end of one year be Rs. 21,600, then B's share in the profit is:

$$\begin{gathered} \text{ Ratio of the initial investment }=\frac{7}{2}:\frac{4}{3}:\frac{6}{5}\Rightarrow 105:40:36 \\ \text{ So the actual investment of }A,B,C=105x,40x\text{ and }36\mathrm{x} \\ \text{ A increases his share }50\mathrm{\%}\text{ after }4\text{ months. Hence the ratio of their investments } \\ =(105\mathrm{x}\times 4)+\left(105\mathrm{x}\times \frac{150}{100}\times 8\right):40\mathrm{x}\times 12:36\mathrm{x}\times 12 \\ =105+\left(105\times \frac{3}{2}\times 2\right):40\times 3:36\times 3 \\ =105\times 4:40\times 3:36\times 3 \\ =35:10:9 \\ \text{ Total }=35+10+9=54 \\ \text{ B's profit }=\frac{10}{54}\times 21600=4000\text{ Rs. } \end{gathered}$$