An asset for drilling was purchased and placed in service by a petroleum production company. Its cost basis is $50,000 , and it has an estimated MV of $12,000 at the end of estimated useful life of 15 years. Compute the depreciation amount in the fourth year and the BV at the end of the fifth year of life by each of these methods:

a. The SL method.

b. The 200 % DB method with the switchover to SL.

c. The GDS.

d. The ADS.