help in non current assets and trial balance

please help me understand the questions..their answers have somewhat confused me…

non current assets[/b]

at 30th september 20X2, the following balances existed ib the records of lambda

plant and equipment cost 860000
accumulated depreciation 397000

during the year ended 30th september 20X3, plant with a written down value of 37000 was sold for 49000.the plant had originally cost 80000.plant purchased during the year cost 180000..it is the companys policy to charge full years depreciation in the year of aquisition and none in the year of sale, using a rate of 10 % on the straight line basis..
what net amount should appear in lambdas statemnet of financial position at 3oth september 20X3 for plant and equipment?
A) $563000
🙂 $467000
C) $510000
D) $606000

the answer to this is C
and the explanation is

COST ACC.DEPRECIATION CARRYING VALU
$000 $000 $000
opening balance 860 397
disposal (80) (43)
purchase 180

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354
960
_____
depreciation 10% 96
____
450
_____
CV 960-450 510

now my question is that y did they calculate depreciation over the total 960000..y didnt they calculate it seperately on every asset..i mean they could have calculated the depreciation on 860000 first and then subtracted that 86000 depreciation from 860000 and then subtracted 37000 from (860000-86000) as 37000 is the written down value of the asset sold..then they could have added 180000 to this value and subtracted depreciation on 1800000…

now another question is on trial balance..the question is
the following is the extract of jims trial balance as at 31st december 20×7

dr cr
recievables 29600
all. for recievables 3100
irrecoverable debts 1600
(i) additional irrecoverable debts of 3000 were discovered at the year end
(ii) it has been decided to make an allowance for recievables of 10% on the adjusted recievables at the year end
the total bad debt expense (irrecoverable and allowance) for the year ended 31st december 20X7 and the closing net recievables balance as at 31st december 20X7 will be

bad debt net recievables
a) 4160 23940
b) 5040 23940
c) 2560 21830
d) 4000 19800

the answer for this is A

irrecoverable debts 1600+3000 4600
decrease in allowance (10% of 29600-3000) (440)
______
total bad debt expense 4160
______

recievables (29600 – 3000) 26600
less allowance for rec. (2660)

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net closing recievables 23940

this is the answer from the book..but according to me..they should have done this

29600- (3000+1600)= 25000 which is the adjusted recievables at the year end..now allowance for deoreciation is 10% of 25000= 2500…so bad debts expense is 1600 + 3000 – decrease in allowance ( 600)= 4000
and net recievables = 29600-(3000+1600) i.e 25000 + 600=25600..
so i think that they shouldve included 1600 figure in every calculation..how wud v know that v dnt have to subtract 1600 from the recievables

now please help me understand bth the questions