I need detailed help on how to solve and where each number comes from, be ready for follow up questions :

A sales budget is given below for one of the products manufactured by Key Co. for the year 2008 :

January : 21,000 units

February : 36,000 units

March : 61,000 units

April : 41,000 units

May : 31,000 units

June : 25,000 units

The inventory of finished goods at the end of each month should equal 20% of the next month's sales. However, on December 31, 2007, the finished goods inventory totaled only 4,000 units. Each unit of product requires three specialized electronic switches. Since the production of these specialized switches by suppliers is irregular, the company has a policy of maintaining an ending inventory at the end of each month equal to 30% of next month's production needs. The requirement had been met on January 1, 2008.

Question : What is quantity of switches to be purchased in March ?

Answer was 154,800 but I really don't get how

Thanks :)