For Pascal's principle, the pressure exerted on the first piston is equal to the pressure exerted on the second piston:
[tex]p_1 = p_2[/tex]
which means
[tex] \frac{F_1}{A_1}= \frac{F_2}{A_2} [/tex]
where F1, F2 are the two forces and A1, A2 the areas of the two pistons.
The areas of the pistons are:
[tex]A_1 =\pi r_1^2 = \pi (5.0 cm)^2=25\pi cm^2[/tex]
[tex]A_2 = \pi r_2^2 = \pi (15 cm)^2 =225 \pi cm^2[/tex]
And the force on the second piston is [tex]F_2 = 20000 N[/tex], so the force that should be applied on the first piston is
[tex]F_1 = F_2 \frac{A_1}{A_2}=(20000 N) \frac{25 \pi cm^2}{225 \pi cm^2}=2222 N [/tex]
