Since None of the Lawn Mowers, ===> Operates for Less than 900 Minutes
Lowest Possible Length: =====> 900 Minutes
Greatest Possible Length: ========> 990
Average Speed ====> 930 (L1 + L2 + L3) / 3 = 930
Set to Lowest Possible: L1 and L2 ======> 900 Minutes Each
Solve: L1 + L2 + L3 ====> (Lawnmowers 1, 2, 3):
( 900 + 900 + L3) / 3 = 930 L3 ====> 990.
Greatest Possible Length of Time; Answer ====> 990 minutes.
Hope that helps!!! : )