JSFiddle - React, Tailwind, and code Playground

HTML

<div><input data-bind="value: a"/></div>
<div data-bind="text: f('simple')"></div>
<div data-bind="text: f('complex')"></div>
<div data-bind="text: c"></div>

JavaScript

function ViewModel() {
    var $t = this;
    this.a = ko.observable(5);
    this.f = function(val) { 
        return $t.a() * (val === 'simple' ? 2: 10);
    };
    this.c = ko.computed(function() { 
        return $t.a() * 20; 
    }); 
};

ko.applyBindings(new ViewModel());