1/5 has been mowed wich is a better estimate for how much lawn he has left now almost all the lawn? Or about half of the lawn

The amount he has left is closer to the whole lawn

than it is to half of the lawn.

- - 0.2 of the lawn has been mowed.

- - He still has 0.8 of it left to mow.

- - The difference between the whole thing and 0.8 of it is 0.2.

- - The difference between half of the lawn and 0.8 of it is 0.3.

- - So he's still closer to needing all of it than needing half of it.

I would say half

1/5 is equal to 2/10 and 2/10 is closer to 5/10 or 1/2 the lawn rather than the 10/10 or the whole lawn.