What is a combination
A combination (nCr) counts how many different ways you can pick r items out of n available, without repeating any and without caring about order, using the formula nCr = n! / (r! × (n − r)!). Picking A, B, and C is the same group as picking C, A, and B.
Combinations with repetition
When the same type of item can be picked several times, like scoops of ice cream in an order with repeated flavors, the total becomes C(n + r − 1, r). Here n is the number of different types and r is how many items you take, regardless of the order they were picked in.
How to work out your odds in a draw
In draws where the order of the numbers does not matter, the number of possible tickets is a combination. If you play a single ticket, your chance of winning is 1 divided by that total. The calculator shows this value as "1 in N" and also as a percentage.
Practical example
In a draw where 6 numbers are picked out of 60, there are C(60, 6) = 50,063,860 possible tickets, so a single ticket has 1 chance in 50,063,860. For a committee of 3 people chosen from 10 candidates, there are C(10, 3) = 120 different committees.