One thing I need to add is that these game (Eternal, Magic as well I think) allow you to have up to four copies of the same card in your deck.
I know nothing about Magic or similar card games, but I saw that mathematics was one of the topics assigned and I was intrigued by the question
When does it make sense to have more than 75 cards?
It sometimes helps to mess around with a smaller or simpler problem and then apply the rule(s) discovered to the actual problem.
Let’s say you have three cards labeled 🦆, 🍕, 🎂
I could label them 1,2,3 or a,b,c but I like using emojis.
If the synergy comes from dealing the 🦆 card first followed by the 🍕 card, then there is 1 out of 6 possibilities that you will get dealt that sequence:
Another way to look at it is that you had 3 possibilities for the first card you drew and two possibilities for the second card you drew.
And 3 times 2 equals 6 combinations. If you had 4 cards, you’d have 4 possibilities for the first card times 3 possibilities for the second card, which would equal 12 combinations.
So 76 cards would equal 76*75=5,700 possible combinations.
Back to the 3 cards. What if the sequence dealt didn’t matter for synergy, that is
🦆🍕 has the same value as
Then in a 3 card deck there are 2 successful draws out of 6 possible combinations. So 2:6 or 1:3.
Your chances are doubled if the order doesn’t matter.
What if your 3 card deck had two of the same card? That is your 3 cards are 🦆🍕🍕.
If order doesn’t matter, you have 4 successful chances out of 6 or 2:3.
So adding another 🍕card increases your chances of a synergy event.
If there are 🍕 cards that can be combined with a number of different cards
to create synergies, then the value of adding the 🍕 card goes up even more.
Hope that was both understandable and somewhat helpful.