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A patent balance of $408 million.

Explanation:

The following can be depicted from the question:

Patent Cost (2018)= $600 million

Less: Amortization for the first three years I.e 2018 - 2020. This will be:

= ($600/20) × 3 = $90 million

Balance as at 2020 = $600 - $90 = $510 million

The remaining patent life will be:

= 8 - 3 = 5 years

Then, Amortization in 2021 will be:

= $510 million / 5

= $102 million

Balance by 2021 will then be:

= $510 million - $102 million

= $408 million