The benefit of Alexa is it's voice recognition abilities, and the ability to choose an appropriate intent based on a voice interaction. If the skill is written with clearly defined intents Alexa will be able to respond as you wanted. It may be that "Calculate..." might be too vague an intent for Alexa to differentiate.
Also, the useful bit is the skill you build. You define how things are calculated, and what answer to give. Unless you are trying to leverage the voice recognition and AI you might be better off going with some other technology (and if you need those things, then maybe WitAI might be more useful to you: https://wit.ai/ it's a little more roll-your-own than Alexa).
Alexa Voice Services (AVS) is available in the US, but not yet the UK or Germany until 2017 (and who know's when for other markets). AVS can be added physical devices that have a speaker and microphone, so it is possible to use Alexa without using an Echo or Echo Dot.
At it's core, the input and output of Alexa apps are JSON (so text). Alexa parses the text response and speaks the appropriate part. I'm not sure that you can route this response in some other way than having it spoken. However, in between the request and response is the Lambda function (or native device function), so in addition to generating the response to Alexa, you could dump the response somewhere else at the same time that would be available outside of Alexa.
