A depreciation schedule for semi-trucks of Ichiro Manufacturing Company was requested by your auditor soon after December 31, 2015, showing the additions, retirements, depreciation, and other data affecting the income of the company in the 4-year period 2012 to 2015, inclusive. The following data were ascertained.

Balance of Trucks account, Jan. 1, 2012
    Truck No. 1 purchased Jan. 1, 2009, cost             $18,000
    Truck No. 2 purchased July 1, 2009, cost               22,000
    Truck No. 3 purchased Jan. 1, 2011, cost               30,000
    Truck No. 4 purchased July 1, 2011, cost               24,000
    Balance, Jan. 1, 2012                                           $94,000

The Accumulated Depreciation—Trucks account previously adjusted to January 1, 2012, and entered in the ledger, had a balance on that date of $30,200 (depreciation on the four trucks from the respective dates of purchase, based on a 5-year life, no salvage value). No charges had been made against the account before January 1, 2012.

Transactions between January 1, 2012, and December 31, 2015, which were recorded in the ledger, are as follows.

July 1, 2012 Truck No. 3 was traded for a larger one (No. 5), the agreed purchase price of which was $40,000. Ichiro Mfg. Co. paid the automobile dealer $22,000 cash on the transaction. The entry was a debit to Trucks and a credit to Cash, $22,000. The transaction has commercial substance.
Jan. 1, 2013 Truck No. 1 was sold for $3,500 cash; entry debited Cash and credited Trucks, $3,500.
July 1, 2014 A new truck (No. 6) was acquired for $42,000 cash and was charged at that amount to the Trucks account. (Assume truck No. 2 was not retired.)
July 1, 2014 Truck No. 4 was damaged in a wreck to such an extent that it was sold as junk for $700 cash. Ichiro Mfg. Co. received $2,500 from the insurance company. The entry made by the bookkeeper was a debit to Cash, $3,200, and credits to Miscellaneous Income, $700, and Trucks, $2,500.

Entries for depreciation had been made at the close of each year as follows: 2012, $21,000; 2013, $22,500; 2014, $25,050; and 2015, $30,400.

Instructions
  (a) For each of the 4 years, compute separately the increase or decrease in net income arising from the company’s errors in determining or entering depreciation or in recording transactions affecting trucks, ignoring income tax considerations.
  (b) Prepare one compound journal entry as of December 31, 2015, for adjustment of the Trucks account to reflect the correct balances as revealed by your schedule, assuming that the books have not been closed for 2015.