david bought 15 gallons of gas. it was 13.60. What operation should i use

Your question is a bit unclear.

Are you wondering, "What operation do I use to find the cost per gallon?"

If this is what you mean, you want to use division.

13.60 ÷ 15 = ?            ---The total cost divided by the number of gallons gives you the cost per gallon

$${\frac{{\mathtt{13.6}}}{{\mathtt{15}}}} = {\frac{{\mathtt{68}}}{{\mathtt{75}}}} = {\mathtt{0.906\: \!666\: \!666\: \!666\: \!666\: \!7}}$$

or rounded to the nearesnt cent, $0.91 per gallon.

That's some pretty cheap gas! I'd fill up your tank at that gas station. :)

The answer is division. If you need to find the cost of one gallon, divide $13.60 by 15 and you will get your answer.

Thanks Ninja and DS,

If you want to know dollars per gallon all you need to know is that per means divide

so it has to be 13.60/15   dollars per gallon.

This one is straight forward but if you make good use of the units you can do some very difficult rate problems easily.

It also extends to chain rule in calculus.