Especially need help for part 4 and 5 of this question

The following is Arkadia Corporation's income statement for last month: The company has no beginning or ending inventories and produced and sold 20,000 units during the month. Required: 1. What is the company's contribution margin ratio? (2 marks) 2. What is the company's breakeven in units? (3 marks) 3. If sales increase by 100 units, by how much should operating income increase? ( 2 marks) 4. If the tax rate is 30%, how many units must be sold to attain an after-tax profit of $84,000? (4 marks) 5. List two assumptions of CVP analysis. (2 marks)