Credit , debit and Test question about Sale Tax , and PP&E

Hi , I have some problems with the ” Credit , ” Debit ” and some test questions here . Please help

1. ” A company purchased a machine for 50,000$ on 1-1×1 . The machine has 5 year useful life and residual value of 5,000 . On 31 Dec 20×2 , 15,000 was spent in ugrading the machine ; Therefore its useful life was extended to 5 years with residual value was expected to be 2,000$ . On 1 July x3 , the asset was revalue at 54,000 $ , residual value remained the same , At the end of year x4 , due to a declined market of the products , the asset was sold at 18,000$ . Caculate the gain/loss on the disposal and extract the FSs at the end of year x4

2. A bussiness commenced with capitak in cash of 1000$ . Inventory costing 800$ plus sales tax is purchased on credit , and half is sold 1000$ plus sales tax , the customer paying in cast at once . The sales tax rate is 20% . What would the accounting equation after these transaction show

And about the increase and decreases of ” Credit , debit ” . I don’t understand it at all . I’m just done from chapter 1 – 3 ( The regulatory framework , The accounting information system and sales tax ) , but i just can understand 30 % of it , but I can’t do maths . Can you help , pls ?
Thank you so much