How do you prove if quantities vary directly?

Step-by-step explanation:

If quantities vary directly, then one is simply the other multiplied by a fixed number. All sets of two numbers in a direct variation are one number multiplied by a fixed number to get the second number.

Example:

1 gallon of gasoline costs $3.50

2 gallons of gasoline cost $7.00

3 gallons of gasoline cost $10.50

Is the price of gasoline a direct variation based on the number of gallons?

3.5/1 = 3.5

7/2 = 3.5

10.5/3 = 3.5

When you multiply 1 gal, 2 gal, and 3 gal by the constant number 3.5 $/gal, you get the price. Since the multiplication is always by the same number, it is a direct variation.