var wrapperEle = document.body.querySelector(".wrapper");

var miles = Number(prompt("What is your estimated miles per year? "));
var gcost = Number(prompt("Current cost of gasoline in your area? "));

function mpgCalc(rate) {
  var rate;
  var estimate = miles/rate*gcost;
  wrapperEle.innerHTML = "To drive a car with an MPG rating of " + rate + " for " + miles + " miles at " + gcost + " per gallon would cost " + estimate;
}

mpgCalc(12);
mpgCalc(17);
mpgCalc(26);
mpgCalc(29);