If your lawn is 21.0 ft wide and 20.0 ft long, and each square foot of lawn accumulates 1350 new snow flakes every minute. how much snow (in kilograms) accumulates on your lawn per hour? assume an average snow flake has a mass of 1.60 mg.

First we can calculate the area of the rectangular lawn using the formula:
Area = Width x Length = 21 ft x 20 ft = 420 square feet
       
And the total number of snow flakes per minute on the entire lawn is:
   
(1350 snowflakes per minute per square foot) x (420 square feet) = 567,000 snowflakes per minute
       
In one hour (or 60 minutes) we get a total of:
   
(567,000 snowflakes per minute) x (60 minutes / 1 hour) = 34,020,000 snowflakes
       
The total mass of which would be:
   
34,020,000 snowflakes x 1.60 mg = 54,432,000 mg = 54.432 kg (as 1 kg = 1,000,000 mg).
       
So 54.432 kg of snow accumulates every hour on the lawn.