Fixed all the issues in the codes

[himank004]

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• I have committed only those files that are required.
• I have tried commit messages concise and reflective of the code committed.
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Detailed description
1.First i took up the addition.py file and then corrected it by replacing the " * " by " + "
and this is the mistake in the addition.py this is a typo error or misplace by fst typing.

2.The GCD is using the same formula as the addition , so i replaced it with the gcd formula which uses a function called gcd .

3.The simple_interest the maximum of the formula is correct but they forget to divide the total product with 100 'and i added the update that is divided by 100 , so this produces the correct output.

Test plan (required)
first tesing the values givrn by the contributor
whch are given they are claiming that it shows an error so i tested the edited code on the same constraints and confirmed that whether the expected output is generated or not.
README.md

Closing issues

closes #1
closes #2
closes #3