User has x amount of probability to get the number he entered

Hi I'm stuck on a probability game.

how can i create a function where user has 20% chance to get the number he inputs? and also he has 15% chance to get previous number and next number?

user can pick a numbers from range 1-20.

Example: If user enters 8. He has 20% chance to get 8. 15% chance to get 7. 15% chance to get 9. etc