Capacitance: How to get 50 microF w/ 80.0 microF

[Z COOL]

Hey gang,
I am doing a Mastering physics question, and it's got me stumped. ><

Q: You need a capacitance of 50.0 microF, but you don't happen to have a 50.0 microfarads capacitor. You do have a 80.0 microF capacitor.

What additional capacitor is needed to get a capacitance of 50 microF?

I was thinking it has something to do w/ C = Q / deltaV. But that would mean that I would need to have some sort of knowledge of Q. I then tried

Ceq = (1/C1 +1/C2)^-1
and solved for 1/C1 to get 49.9875 microF. But that doesn't seem logical (+ I got it wrong ><)

Any suggestions / help? Greatly appreciated!

 

[ranger]

Well you obviously solved for the unknown incorrectly. Why not try to solve the simpler equation (assuming series capacitors):

C_{eq} = \frac{C1 \cdot C2}{C1+C2}