GrandAuto sells only two items: Tyres and Engine Oil. Tyres Engine Oil Selling Price per unit ($) 250 50 Required: Variable Cost per unit ($) 100 20 The total fixed cost per month is $30,000. (a) Calculate the number of units GrandAuto would need to sell to generate a monthly profit that is double of the monthly fixed cost if it sells Tyres only. (3 marks) (b) Calculate the expected monthly profit/loss GrandAuto would generate if it sold 100 units of Tyres and 500 units of Engine Oil. (3 marks)