Is it possible to add interactive calculator within Marketo landing pages and then perform some calculation based on inputs received from various questions within the landing page? For e.g. there are two multiple choice questions and final calculation will be done based on selection through these questions. If this functionality is not available within Marketo so is there any third party tool that can be integrated with Marketo?
Sure, you can build a calculator directly on a Marketo form using bespoke code and <input type=range> and/or other markup. Of course you'd need a Marketo-fluent developer, but you can make great-looking stuff that way and be assured of Marketo compatibility (since the underlying form is still a Marketo form).
Or you could use some kind of standalone calculator builder/widget, provided it has an API that allows you to submit the form using the Marketo Forms API.
Though your example of "two multiple choice questions" hardly sounds like it needs a calculator. You just need some JS to add up the values of two fields or what-have-you and then submit (and display?) the total value in another field.