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