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:

Example

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.

Source link for the following key concepts.

Direct Variation – online worksheet link

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