Suppose a car can run on ethanol and gas and you have a 15 gallons tank to fill. You can buy fuel that is either 30 percent ethanol or 80 percent ethanol. How much of each type of fuel should you mix so that the mixture is 40 percent ethanol?  Choose one
A) 11	B) 12	C) 13	D) 14	E) 15
Le x represent number of gallons of gas that contain 30 percent ethanol   Let 15 - x be number of gallons of gas that contain 80 percent ethanol   Since the mixture contains 40 percent ethanol, only 40% of the 15 gallons will be ethanol   40% of 15 = (40/100) times 15 = (40/100) times 15/1 = (40 × 15) / (100 × 1) = 600 / 100= 
<gadget id="calculator">600 / 100</gadget>
<output>6</output>
6   In order for x gallons of gas to contain 30% ethanol, we must take 30% of x or 30% times x   In order for 15 - x gallons of gas to contain 80% ethanol, we must take 80% of 15 - x or 80% times 15 - x   0.30 × x + 0.80 × (15 - x)= 6   0.30x + 0.80 × 15 - 0.80 × x = 6   0.30x + 12 - 0.80x = 6   0.30x - 0.80x + 12 = 6   -0.50x + 12 = 6   -0.50x = -6   x = 12   So 12 gallons of gass contain 30 percent ethanol and 15 - 12= 
<gadget id="calculator">15 - 12</gadget>
<output>3</output>
3 gallons contain 80 percent ethanol   Therefore, mix 12 gallons of a 30% ethanol with 3 gallons of an 80% ethanol   Answer 12
<result>B</result>
Suppose a car can run on ethanol and gas and you have a 15 gallons tank to fill. You can buy fuel that is either 30 percent ethanol or 80 percent ethanol. How much of each type of fuel should you mix so that the mixture is 40 percent ethanol?