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 weather sal will be able to afford 4 gallons of gas ?

In order to make an equation to find out whether or not he can afford the gas,, we must know that since $3.29 is changing so it must have a variable. Our equation also needs to equal to $15,, so it will look like the following:
15 = 3.29x
Before we start solving,, we need to plug the 4 gallons in for x.
15 = 3.29(4)
Now multiply the numbers on the right side.
15 = 13.16
This show us that the cost of 4 gallons is less than $15,, which means that Sal will be able to afford 4 gallons of gas.
Let me know if you have any further questions.
:)