(If you read the box on expressing power ratios in decibels) If a power ratio is 20:1, this can be expressed as 10:1 times 2:1. You know that 10:1 is 10 dB and that 2:1 is 3 dB. Although power ratios multiply, decibels add. So, a power ratio of 20 (10 times 2) is 10 + 3 dB or 13 dB. Learning from this example, what will a power ratio of 1:800 be in decibels?

Can you please explain how you get the answer? I am lost. Thank you.