Sal wants to know if he has enough money to buy 4 gallons of gas. If he has $15, and each gallon of gas costs $3.29, which is a good estimate for whether Sal will be able to afford 4 gallons of gas?$3 + $3 + $3 + $3 = 12, so Sal can afford the gas. $4 + $4 + $4 + $4 = 16, so Sal can afford the gas. $3 + $3 + $3 + $3 = 12, so Sal cannot afford the gas.$4 + $4 + $4 + $4 = 16, so Sal cannot afford the gas.

Remember that when you are making stimates, you are going to roiund to the nearest dollar. Since $3.29 is closer to 3 dollars than four dollars, Sam should use the 3 dollars figure to estimate if he can afford 4 gallons of gas.
$3+$3+$3+$3=$12 
The actual value is [tex]4(3.29)=13.16[/tex], so $3 per gallon is indeed a god estimate.

We can conclude that the correct answer is: $3 + $3 + $3 + $3 = 12, so Sal can afford the gas.