Fix all issues

[PramodToleti]

Your checklist for this pull request

• I have committed only those files that are required.
• I have tried to commit messages concisely and reflective of the code committed.
• I have used a branch name reflective of the issue.
• If there is a relevant issue for the PR, I have used closes #<issue number> to auto-close the issue once PR is merged.

Detailed description

Fixed Addition not working #1

• Fixed typo in the addition.py file
• Replaced '*' with '+' to perform the addition operation

Fixed Greatest Common Divisor not working #2

• Fixed issue in the gcd.py file
• Imported math library to find the GCD of 2 numbers

Fixed Simple Interest Calculator gives wrong output #3

• Fixed issue in the simple_interest.py file
• Divided the product of principal, rate, and time by 100 to get the interest

Test plan (required)

Closing issues
closes #1
closes #2
closes #3