If the width of the lawn is x, the length and width of the flower planting part are divided into 50-X and 40-X respectively.

So the area is (50-x) (40-x) = x2-90x+2000 = 0.3 * 2000 = 600.

Sort out the equation: x2-90x+ 1400=0.

(x-20)(x-70)=0

The solution is x=20 or x=70 (if it exceeds the width of the green space, the solution is discarded).

So the width of the lawn is 20 meters.