The statement ” y varies directly as x ,” means that when x increases,y increases by the same factor. In other words, y and x always have the same ratio:
If a gallon of milk costs $3, and I buy 1 gallon, the total cost is $3. If I buy 10 gallons, the price is $30. In this example the total cost of milk and the number of gallons purchased are subject to direct variation — the ratio of the cost to the number of gallons is always 3.
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