In the given figure mass of string AB is 2Kg. Find tensions at A, B and C, where C is the midpoint of string.

$$\begin{gathered} a=\frac{F\text{ - weight of }2 \mathrm{kg}\text{ - weight of }4 \mathrm{kg}\text{ - weight of string }}{\text{ mass of }2 \mathrm{kg}+\text{ mass of }4 \mathrm{kg}+\text{ mass of string }} \\ =\frac{100-20-40-20}{2+4+2} \left(g=10 \mathrm{m}/{\mathrm{s}}^2\right) \\ =\frac{20}{8}=25 \mathrm{m}/{\mathrm{s}}^2 \end{gathered}$$

$$\begin{gathered} T_A-m_{AB}g-40=\left(m_{AB}+4\right)a \\ {T}_A-20-40=(2+4)(2.5) \\ {T}_A=75 \mathrm{N} \\ {T}_C-m_{BC}g-40=\left(m_{BC}+4\right)a \\ {T}_C-10-40=(1+4)(2.5) \\ {T}_C=62.5 \mathrm{N} \\ {T}_B-40=4a \\ {T}_B=40+4\times 2.5 \\ {T}_B=50 \mathrm{N} \end{gathered}$$