F3, BPP exercise

Dear Sir,
I can’t seem to understand the answer to this exercise:
The trucks account (at cost) of a business for the year ended 31 December 20X1 was as follows:
TRUCKS – COST
20X1 20X1
$ $
1 Jan Balance 240,000 31 Mar Disposal account 60,000
30 June cash – purchase of vans 160,000 31 Dec Balance 340,000
400,000 400,000
Brought forward accumulated depreciation at 1 January 20X1 was $115,000. The truck
disposed of on 31 March had a carrying value of $20,000.
The company’s policy is to charge depreciation at 20% per year on the reducing balance, and
charges a full year’s depreciation in the year of acquisition and none in the year of disposal.
What should be the depreciation charge for the year ended 31 December 20X1?

and the answer from the book is:
Charge for the year
Trucks at 1 Jan $’000 $’000
Cost (240 – 60) 180,000
Acc dep’n (115 – (60 – 20)) 75,000
Carrying value 105,000
Dep’n charge @ 20% 21,000 21,000
Purchased truck
Dep’n charge @ 20% 160 @ 20% 32,000 32,000
53,000

I don’t understand why the cost is 240-60 and not 240; Also, why from acc depr is deducted 20? the carrying value shoudnt be 240-115 ?