gistfile1.txt

var calcIndividual = function (input) {
          if (input < 6001) {
            var output = 0;
          }
          if (6000 < input < 35001) {
            var output = ((input - 6000) * 0.15);
          }
          if (35000 < input < 80001) {
            var output = (4350 + ((input - 35000) * 0.3));
          }
          if (80000 < input < 180001) {
            var output = (17850 + ((input - 80000) * 0.38));
          }
          if (input > 180000) {
            var output =(55850 + ((input - 180000) * 0.45));
          }
          return output;
        };

Number.prototype.between = function(a, b) {
    return (this >= a && this <= b) ? true : false;
}
alert(new Number(10).between(5, 7)); // false

You have bugs. The values 6000, 35000, 80000 and 180000 will return bogus output.