Developing the cash flow for each alternative in a study is a pivotal, and usually the most difficult, step in the engineering economic analysis procedure. An integrated approach for developing cash flows includes three major components: a. A work breakdown structure (WBS) definition of the project.
b. A cost and revenue structure that identifies all the cost and revenue elements involved in the study
c. Estimating techniques (models).

Required:
Discuss the concept of equivalence - if two cash flows (or a series of cash flows) are equivalent for a stated interest rate, and under what circumstances you would be willing to trade one for the other.

The concept of equivalence, also known as economic equivalence, describes the reduction of a series of cash inflows (benefits) and cash outflows (costs) to a single point in time, using a single interest rate, which enables the cash flows to be compared or equated.  This implies that while the amounts and timing of the cash flows (both inflows and outflows) may differ, an appropriate interest rate, factoring in the time value of money, will cause one set to be equal to the other.  Therefore, to establish economic equivalence, series of cash flows that occur at different points in time must be equalized using a single interest rate through present value calculations.

Explanation:

The concept of equivalence describes a combination of a single interest rate and the idea of the time value of money.  This combination helps to determine the different amounts of money at different points in time that are equal in economic value, such that a person would not hesitate to trade one for the other.

For example, if the interest rate is 10% in Year 1 and in Year 2 and you are to be paid $1,000 in Year 1, it will not make any difference to you if you are paid $1,100 in Year 2.  This is because, given the prevailing interest rate of 10%, the value you receive in Year 1 and Year 2 are equivalent.

(75years ,i believe is the correct answer.good luck