Where to find an example of a simple calculator calculate the amount?

Tell me a simple example of a calculator, where is the usual input where you enter the number, it is multiplied by 2 or 3 etc
and displays the final figure in the usual <Ddiv>
March 23rd 20 at 18:40
1 answer
March 23rd 20 at 18:42
There can be various implementations to find. Please.
https://codepen.io/alexboffey/pen/NAkQLE

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