How to make calculations with javascript or jQuery, grabbing values from various different elements?
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I am currently attempting to build a quote calculator for my first website, I have been learning Javascript for a week, I have used jQuery to hide various elements when the website loads then show upon clicking relevent buttons. First Q, by doing this, am I creating any conflict between any new javascript code and the jQuery code? 

I have built a pop-out interface that breaks down services offered, there are 5 categories. I have further developed one category to show, when clicked 'How many boxes do you need to store?' with a textbox beneath it and a submit button next to it. On clicking the submit button it shows a new question; 'How long would you like to store for?' and two more textboxes named 'weeks' and 'months' with another submit button. 

What I'm hoping to achieve is to create a function that grabs values entered into the first box, divides the given number by 18 and round it UP to the next whole number, even if its 1.000001. Simultaneously, I need the value entered into 'months' to be multiplied by 4.33 and rounded UP aswell. Then add the result from months*4.33 to the value entered into weeks to give total duration, and the final calculation must be (totalduration*2.50)*resultofthefirstcalculation. The final result must then be displayed in a textbox of my choosing as a monetary value - £0.00

Many questions:

Do all the textboxes I'm using have to be within <form> tags for javascript to find them properly?
If so, does it all have to be in one form or can I use more than one?  
Can I mix Javascript and jQuery within the same <script> tags? If so, should one go before the other?

Later on, once the quote calculator is functional, I plan to use PHP to send an email to the user with their quote details, and to myself with the same. Do I need to make considerations for this in my HTML/Javascript? Do all the details I want to send have to be in: one form/more than one form/doesn't matter, can be any element?