Let S(x) represent the total sales (in thousands of dollars) for the month x in the year 2005 at a certain department store. Represent each following statement by an equation involving S or S′. 

(a) The sales at the end of January reached $120,560 and were rising at the rate of $1500 per month. 

(b) At the end of March, the sales for this month dropped to $80,000 and were falling by about $200 a day.