Can some one please share your thoughts on these

A task was scheduled to use two persons, full time, and take two weeks to complete.Instead, the project manager was only able to assign one person to this task. At the endof two weeks, the person assigned to the task was 75% complete. What is the costperformance index?
a. 0.75
b. 1.5
c. 1.33
d. .5
Answer B how ?
A project manager and the project team identify several specific risks in a project. The expected value of these risks is estimated at $10,000. The impact on the project brought about by these risks is estimated at $40,000. What value should be entered into the contingency reserve for these risks?
a. $10,000
b. $40,000
c. $0
d. $25,000

Answer A , I thought B ,pls explain

The time-phased cost of the project that reflects the expenditures rather than the consumption of a resource that will be used to measure and monitor cost performance on a project is the:
a. Spending plan
b. Cost baseline
c. WBS
d. Schedule
Answer is A,  I thought B

 A large piece of equipment is needed for the project. It will be retained and used after the project is completed. The equipment has a value of $500,000, and it has a scrap value of $50,000 at the end of its useful life of ten years. If the sum of the years’ digits method of depreciation is used, what is the depreciation that is taken at the end of the third year?
a. $81,818
b. $90,900
c. $65,250
d. $72,500

A project manager is considering applying learning curve theory to his project. Theproject involves designing a number of software modules that are very similar. Accordingto the cost figures that have been collected the first unit required 100 person-hours todesign and test. The second unit required 90 person-hours to design and test. Howmany person-hours will the eighth module take to design and test?
a. 90
b. 100
c. 73
d. 172

Answer is C , I have no clue how it is calculated, can some one explain formula for learning curve.