Black Bulls And White Cows

Remember the classic Master Mind game from the Seventies?
It is also called "Bulls and Cows".
You might know this game from your childhood as it is a digital version of the classic Mastermind board game.
Now it's available for FREE for your phone and tablet.
The goal is to guess the hidden digital combinations.
Your opponent (the computer / phone) creates a combination of up to four digits and hides it at the top of the game board.
Your task is to try out different digital combinations until you match the hidden code.
After each try, you get a hint to see how many numbers you have correct:
-Black key: There is a digit that is in the combination and at the right place.
-White key: There is a digit that is in the combination but not at the right place.
The challenge is to pick the correct numerical code as quickly as possible — in other words,
to see how fast you can guess the hidden color combination.
Its a very addictive puzzle game.
There are 3 categories of difficulty and 6 groups by dimension (3 pins, 4 pins, 5 pins, 6 pins, 7 pins, 8 pins).
Each categorie has 50 - 1000 levels.
There are more then 10000 tasks.

Rules:
https://en.wikipedia.org/wiki/Mastermind_(board_game)