Assume this piston has a maximum pressure rating of 2.5 pounds-force per square inch [psi]. The force applied to the cylindrical piston is 100 newtons [N]. What is the diameter of the piston, in units of inches [in]

Explanation:

Always write what you know first about the problem

The surface of the piston that has the force applied is a circle

The area of a circle is pi x r^2

Pressure is Force / Area

Write this out

P = F / (pi x r^2)

solve for r

sqrt( F / (P x Pi)) = r

But dont forget that d = 2 x r