please need help in this. here is the response but do not understand why it is dividing by 3

then by 4, could you explain in a simple way?

i will really appreciate your help on this!

Panther Company purchased a machine for $200,000 on January 1, 2018. Panther Co. depreciates machines of this type by the straight-line method over a 10 -year period with the residual value of $20,000. Due to a change in sales patterns, on January 1,2021, management determines the useful life of the machine to be a total of seven years. What amount should Panther Co, record for depreciation expense for 2021? $31,500$18,000