Fixing issues

[meihu-ganesh]

Your checklist for this pull request

• I have committed only those files that are required.
• I have tried to make commit messages concise 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

• In the Issue Addition not working , inside addition.py file there was a small bug where multiplication is used instead of addition.
• in the issue Greatest Common Divisor Not working , inside gcd.py file a package math is imported and direct gcd function is used from the package.
• in the issue Simple Interest Calculator gives wrong output , inside simple_interest.py file the formula used was wrong in the line 7 due to missing of '/100' for the value obtained.

Test plan (required)

#1 Addition not working after resolving issue:

#2 Greatest Common Divisor Not working after resolving issue:

#3 Simple Interest Calculator gives wrong output after resolving issue:

Closing issues

closes #1 #2 #3