Q. A truck purchased for $20,000 has accumulated amortization of $6,000 on January 1st and is amortized at a rate of 10%. What's the entry for 6 month period ending June 30th?

What is the adjusted journal entry for the 6 month period ending June 30th?

Furthermore the amortization formula for the declining method is as follows (as you know)

Amortization expense = (Purchase price - Accumulated Amortization to date) * Amortization rate

Please help me with the explanation of the formula aswell using this question