This from Wikibooks:
Vexternal = E - Vinternal
Since V = IR:
Vexternal = E - IRinternal
You may also need to use the following formula to work out the external potential difference, if you are not given it:
Vexternal = IΣRexternal
You should also remember the effects of using resistors in both series and parallel circuits.
The reason I'm posting this here is to remind me to learn it !!