WritingEquations(2)

　　Gus needs to paint his house, which has a surface area of x square feet. The brand of paint he buys (at a cost of p dollars a can) comes in cans that cover y square feet each. Gus also needs to buy ten pairs of new jeans (he is uncoordinated and spills often). They cost d dollars a pair. If Gus makes these purchases, what is the difference (in dollars) between the cost of the paint and the cost of the jeans? Assume he doesn’t buy any excess paint—that is, the required amount is not a fraction of a can.

　　This word problem is long and complicated, but you need to carry out just four steps to solve it:Gus must buy x/ y cans of paint to cover his house. This will cost him xp/ y dollars. The jeans Gus buys cost 10d dollars. Thus, the difference, in dollars, between the cost of the paint and the cost of the jeans is xp/ y – 10d.　　For the rest of this chapter, we’ll constantly be converting word problems into equations. If you’re still uncomfortable doing this, don’t worry. You’ll get a lot more practice in the sections to come.

（责任编辑：申月月）