24 minutes.
Let me explain.
The lawn can be any size, but for the purpose of simplicity, let's say it's 240 square meters.
Peter mows 6 square meters per minute if he mows 240 square meters in 40 minutes.
John mows 4 square metres each minute if he mows 240 square metres in 60 minutes.
They'll mow 10 square meters per minute if they work together.
As a result, mowing the entire lawn will take them 24 minutes.
(Note that the location is irrelevant as long as we know how long each man takes to complete the task separately.) The answer would be the same if the field was 1 million hectares in size.)
I hope this helps.