Edit in JSFiddle

var taxRate = 1.2;
var items = [20, 25, 17];

var total = _.reduce(items, function (runningTotal, value) {
    return runningTotal + value * taxRate;
}, 0);

console.log(total);

              
            
          
            
              
                

              
            
          
            
              

External resources loaded into this fiddle: