Mia buys 3 gallons of gas at d, or 3 * d = 3d. The number <em>3</em> represents the quantity(how many) gallons she bought The variable <em>d</em> represents the rate at which Mia bought the gas.
Bob buys g gallons at 3, or 3 * g = 3g. The variable <em>g</em> number 3 represents the quantity(how many) gallons he bought The number <em>3</em> represents the rate at which Mia bought the gas.
3(d) and 3 (g) d represents the price. so you multiply 3 by however much the has cost. g represents the gallons and 3 represents the cost and again you multiply