I'm confused about the formula for the NPV of the older equipment in the Gargle Blaster example in the textbook:

"NPV= - (investment) + PV(price - manufacturing cost)

= -2.50 +(5.75 - 5.50) / 0.20"

I know that the opportunity cost of capital is 20% but I don't understand why the second term on the right-hand side of the equation is divided by "0.20". 

[NEAS: Formula for present value of an infinite series of equal cash flows. The present value of 1 each period forever is 1/(discount rate).]