JSFiddle - React, Tailwind, and code Playground

HTML

<input data-bind="value: var1" type="text">
<input data-bind="value: var2" type="text">

JavaScript

function DataViewModel() {
        var self = this;
        self.price = 5;
        self.var1 = ko.observable(0);
        self.var2 = ko.computed({
            read: function () {
                if(self.var1() <= 0) {
                    return 0;
                }
                return (self.var1() - 1) / self.price;
            },
            write: function (value) {
                self.var1((Number(value)+1) * self.price);
            },
            owner: self
        });
    }
ko.applyBindings(new DataViewModel());