In: Advanced Math
There are certain number of balls and they are painted
with following conditions:
1. Every two colours appear on exactly one ball.
2. Every two balls have exactly one colour in common.
3. There are 4 colours such that any three of them appear on one
ball.
4. Each ball has 3 colours.
Find number of balls and number of colours used.
(Answer: 7 balls and 7 colours)