How to write a script that sticks to the formula number depending on value?

To register a script that substitutes in the formula, the number depending on the value
The script multiplies the value with the value of likes or members, where likes = 2 and members=3.
Once in the value of the digit can substitute, as it is needed for further operation of the program.
<select class="type">
 <option value="likes">Likes</option>
 <option value="members">Members</option>
<input type="number" class="amount" placeholder="Quantity">
<div class="date"></div>

In JS still a newbie, so asking for help from you....
June 14th 19 at 20:53

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