How Long for Potential Difference to Reach 50 Volts in an Oscilloscope?

[sdevoe]

Homework Statement

The deflection plates in an oscilloscope are 10 cm by 2 cm with a gap distance of 1 mm. A 100 volt potential difference is suddenly applied to the initially uncharged plates through a 900 ohm resistor in series with the deflection plates. How long does it take for the potential difference between the deflection plates to reach 50 volts?

Homework Equations

q=c*(delta_V)

The Attempt at a Solution

There has to be an equation that I am missing or need

 

[rl.bhat]

The whole problems converts into a RC circuit.
Find the expression for the charging of the capacitance.