Depreciation and disposal

At 30 September 20X2, the following 
balances existed in the records of Lambda Co:  Plant and equipment: 
Cost  $860,000  Accumulated depreciation  $397,000  During the year ended 30 September 20X3, plant with a written down value of 
$37,000 was  sold for $49,000. The plant had originally
 cost $80,000. Plant purchased during the year cost $180,000. It is Lambda Co’s 
policy to charge a full year’s depreciation
 in the year of acquisition of an asset and none in the year of sale, using a rate of 
10% on the straight line basis.  What was the carrying amount that should appear in Lambda Co’s statement of
 financial position at 30 September 20X3
 for plant and equipment? 

Sir the answer is 510,000

Why can’t we calculate the answer by finding the carrying amount of total assets then deducting carrying amount of disposal then adding carrying amount of the new asset???
(463000-37000+16200) =588000

When we dispose an asset do we deduct the carrying amount or cost from the non current asset account?