A depreciation schedule for semi-trucks of Murph Manufacturing Company was requested by your auditor soon after December 31, 2021, showing the additions, retirements, depreciation, and other data affecting the income of the company in the 4-year period 2018 to 2021, inclusive. The following data were ascertained.

Balance of Trucks account, Jan. 1, 2018
Truck No. 1 purchased Jan. 1, 2015, cost		$21,600
Truck No. 2 purchased July 1, 2015, cost		26,400
Truck No. 3 purchased Jan. 1, 2017, cost		36,000
Truck No. 4 purchased July 1, 2017, cost		28,800
Balance, Jan. 1, 2018		$112,800

The Accumulated Depreciation-Trucks account previously adjusted to January 1, 2018, and entered in the ledger, had a balance on that date of $36,240 (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, 2018. Transactions between January 1, 2018, and December 31, 2021, which were recorded in the ledger, are as follows.

July 1, 2018		Truck No. 3 was traded for a larger one (No. 5), the agreed purchase price of which was $48,000. Murph. paid the automobile dealer $26,400 cash on the transaction. The entry was a debit to Trucks and a credit to Cash, $26,400. The transaction has commercial substance.
Jan. 1, 2019		Truck No. 1 was sold for $4,200 cash; entry debited Cash and credited Trucks, $4,200.
July 1, 2020		A new truck (No. 6) was acquired for $50,400 cash and was charged at that amount to the Trucks account. (Assume truck No. 2 was not retired.)
July 1, 2020		Truck No. 4 was damaged in a wreck to such an extent that it was sold as junk for $840 cash. Murph received $3,000 from the insurance company. The entry made by the bookkeeper was a debit to Cash, $3,840, and credits to Miscellaneous Income, $840, and Trucks, $3,000.

Entries for straight-line depreciation had been made at the close of each year as follows: 2018, $25,200; 2019, $27,000; 2020, $30,060; 2021, $36,480.

(a) For each of the 4 years, compute separately the increase or decrease in net income arising from the companys errors in determining or entering depreciation or in recording transactions affecting trucks, ignoring income tax considerations.