So we know that 2 workers can make 2 chairs in 2 days. Therefore if we use that same proportion, we will find that 8 workers can make 8 chairs in 2 days.
But the question is asking for 20 days, so we must multiply this by 10.
So, when we multiply 8 by 10, we find that in 20 days, 8 workers can make 80 chairs.