How can I perform calculations in my form?

Hi I would like to make a form where 

Once the user selected

Option A  qty = 10  then it should calculate $2*10 =  $20

Option B  qty = 10  then it should calculate $5.00*10 =  $50

Showing a total of $70 in the calculated Field

This is some sort of a form to inform the end user with a price for his order

Yes, this is indeed possible, you only need to add the needed fields to display your option as well as the quantity:

To the option in the check box you will need to add a calculation value, this will be like the individual price: 

Then you only need to add the calculation field to your form: 

Once added you only need to perform your calculation lie shown on the screenshot below:

If you would need to add another calculation then you would only need to add it like this "+(OtherCheckbox * QuantityForThisOption)". 

Here's also a sample form where you can see how this works: https://form.jotform.com/70387026387967 

Here're also some guides that will help you: 

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field

https://www.jotform.com/help/301-How-to-Assign-Calculation-Value 

If you have any question, let us know.