Let’s take EUR/USD as example?
Do you mean, why EUR/USD price is equal X and not Y? 
I will assume, that yes and… won’t answer your question.
Currency pair price is the same as exchange rate for EUR and USD (how many USD you can buy for 1 EUR)
Currency alone does not have “a price” - we can only assess value from perspective of other currency. So currency pair is a reflection of strength between two economies (in short). In longer term this is driven by underlying economies, in shorter term by simple law of supply and demand.
Usually, I use cars as an example, which is easier to grasp.
You cannot CALCULATE the price of used car. We can assume that 10 years old Volkswagen would cost more than 10 years old Ford. We may also try to understand why - maybe better rust protection, german cars overall had good reputation etc. (nothing personal to one of Mondeo fans around here). But can you calculate what EXACT rate of price between Ford and VW should be? Definitely not. You can see price fluctuations and rate would change (eg. higher demand for one car or another), but there is no absolute reference to use for calculation. We can only do this by comparing one to another - getting the “exchange rate” as it is. The same goes to economies and currency pairs. You can look at some macroeconomic factors and say “yeah, I expect euro to get stronger against US dollar” but you can’t say “looking at employment rates EURUSD definitely should be 1.17”
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Hi @SmartManoj,
Your questions are very very basic, and also incomplete. Please start at this link and do some reading. If you continue with more very very basic questions before reading up on the subject you will not get any answers to more serious questions.
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Hellooo. 
Although I’m sure you’re just eager to get started, I would have to agree with the others and suggest that you read the school here first. 
It will give you a good grasp of the basics, and maybe help you be more specific with your question. And I guess once you’re done with that, you can also explore using some tools to help you with your calculations.
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