What is the mass of snow (in tons) on a 150.0ft by 45.0 ft flat roof after a 6.0 inch snowfall? assume that 11 inches of snow is equivalent to 1.0 inches of water. the density of water is 1.00g/ml.

6 inches of snowfall would be equivalent to 6/11 inches of water or 6/132 feet. So the volume of the equivalent amount of water on the roof is 150 ft. x 45 ft. x 6/132ft. = 306.8 cubic feet.   
Since there are 907185 grams in a ton and 28316.8 mL in a cubic foot, 1.00 g/mL is equivalent to 0.0312 tons/cubic foot  
Multiplying the two results gives 9.58 tons of snow.