What is the Height of the Step Potential if 25% of a 9 eV Beam is Reflected?

[Helmholtz]

Homework Statement

A beam of particles of energy E = 9 eV strikes on a step potential and 25% of the beam's particles is reflected back. What is the height of the step potential?

(A) 4 eV
(B) 8 eV
(C) 3 eV
(D) 1 eV
(E) 5 eV

Homework Equations

√(R) = [√(E)-√(E-U)]/[√(E)-√(E_U)]

The Attempt at a Solution

Just plug and chug the equation I have, but this is for a timed test without a calculator or equation sheet. This seems clunky to remember, solve and do in such conditions. Is there a simpler way? I get 8eV, which is the correct answer.

 

[mfb]

You can remember that you need a significant potential step to get significant reflection.
Estimate reflection for 5 eV, if the value is too low, it has to be 8 eV.

Is there a sign error in your formula?