Let forces $\rm F_{A}$ and $\rm F_{B}$ are acting on the two pistons A and B, respectively. Similarly. let $\rm A$ and $\rm B$ be the surface area of each pistons.

Given,

$\rm F_{A} = 20 N$ and $\rm A = 0.5 cm^{2}$

$\rm F_{B} = ? N$ and $\rm B = 5 cm^{2}$

By Pascal's law of liquid pressure, we have

$\rm P_{A} = P_{B}$

$\rm or, \frac{ F_{A}}{A} = \frac{F_{B}}{B}$

$\rm or, F_{B} = \frac{ B}{A} \cdot F_{A}$

$\rm or, F_{B} = \frac{ 5}{0.5} \cdot 20 N$

$\rm \therefore F_{B} = 200N$

Hence, the required force to be applied through syringe B to balanced the force on piston A is 200 N.