If I use 6 different coloured yarns to mark my pea crosses and either use one colour or maximum two colours together to mark different crosses, how many different combinations do I get with my yarns?

Just spent half an hour working out that I have 21 combinations on the back of an envelope. And what combinations I have still available as I have already done several crosses and got fed up looking down my sheet every time to check I hadn't used a combination already..

There must be a mathematical way of doing this that is much quicker

Just spent half an hour working out that I have 21 combinations on the back of an envelope. And what combinations I have still available as I have already done several crosses and got fed up looking down my sheet every time to check I hadn't used a combination already..

There must be a mathematical way of doing this that is much quicker

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