Ratio of trees to grass for healthy lawn?

What would be a good ratio of trees to grass to have a healthy lawn? Our neighbors have a ton of trees on their 1.5 acre lot and to me it would seem that it's overcrowded and not good for either the trees or lawn to get enough water and sun?
Grass can be easily grown under shades because sun's heat can spoil the grass. You may seen in Gardens where the grassi good with clean and green.For watering grass use sprinkler that can spray enough water on the grass.