This the same question as "What does 'hindsight is 20/20' mean", which has already been answered.
It mean that you can look in the past (hindsight) and see what you did wrong. Perfect vision is 20/20. So as you look back, you're able to see "perfectly" what you should have done. "Hindsight is...
8,000
The answer is 60 although it could also be 8000
A nanometre is equal to one billionth of a metre. Therefore, 20 nanometres is equal to 20 billionths of a metre.