Assets: JAN o1=230
Liabilities; jan 01=80
OE jan 01=150
assets dec 31=310
liabilities dec 31=90
oe dec 31=220
revenues = 400
expenses=?

First you have to calculate your ending Owners Equity by taking your ending Assets minus Ending liabilities.

Now you need to get the difference between your beginning Owners Equity plus Revenues and the Ending Owners Equity calculated above. This difference is your Expenses.