assets = liabilities + owner's equity, owner's equity = capital + retained earning, 所以 retained earning = 300,000-160,000-100,000 = 40,000
又因为dividend 不属于expense , 同时 revenue and expense account will be closed to the retained earning account, 所以 retained earning = revenue - expense - dividend 所以expense = 400,000- 40,000 - 20,000 = 340,000