Tangible non-current asset

Dear sir,I want to know how to calculate this question?What is the “plant held all year?Why it is (200000-40000)*20%?
Right Co’s policy is to charge depreciation on plant and machinery at 20% per year on cost, with proportional depreciation for items purchased or sold during a year.
Right Co’s plant and machinery at cost account for the year ended 30 September 20X3 is shown below.
Dr:1.10.20×2 balance is 200000
1.4.20×3 Cash-purchase of plant is 50000
Cr:30.6.20×3 Transfer disposal account is 40000
30 Sep.Balance is 210000
What should be the depreciation charge for plant and machinery (excluding any profit or loss on the disposal) for the year ended 30 September 20X3?
Thank you very much!