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

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.

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.