Answer
6.18
Work Step by Step
Let's take,
Force applied to the input piston = $F_{1}$
Force of the output plunger = $F_{2}$
We know that,
Pressure at the output plunger = Pressure at the input plunger
So, we can write,
$\frac{F_{1}}{A_{1}}=\frac{F_{2}}{A_{2}}$ ; Let's plug known values into this equation.
$\frac{55\space N}{\pi r_{1}^{2}}=\frac{2100\space N}{\pi r_{2}^{2}}=>\frac{r_{2}}{r_{1}}=\sqrt {\frac{2100}{55}}=6.18$
