Calling card - DocTest

You buy an international calling card to India. The calling card company has some special offers.

• If you charge your card with $5 or $10, you don't get anything extra.
• For a $25 charge, you get $3 of extra phone time.
• For a $50 charge, you get $8 of extra phone time.
• For a $100 charge, you get $20 of extra phone time.

Write a function that asks the user for the amount he/she wants on the card and returns the total charge that the user gets.
Note: Values other than those mentioned above are not allowed.