pity that the deckbuilder gets the answer wrong
..
It looks like it is using a 63 card deck for its calculation of having a basic to start with.
BTW I found that I did have a MyNintendo account even though I'm a foreigner. It seems that there was a loophole in the signup procedure some months ago whereby you didn't HAVE to provide an address. So as long as I don't edit MyNintendo details I should be able to log in for a while.
The probability of not muliganing is = (1-probability_of_starting_with_none)
so for 6 basics it is 1- 54/60*53/59*52/58*....*48/54 which is 0.5414
(this is 1-hypgeomdist(0,7,6,60) in Excel
ie 1-hypgeomdist(in_hand,hand_size,number_in_deck,deck_size) )
very roughly you muligan half the time with 6 basics.
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Be careful how you calculate the probability of starting with just a single basic. Since you are STARTING you have to remove all those cases where you did get zero basics .
Staying with six basics for now these are the results from the hypergeometric distribution
H(0,7,6,60)=0.458563922
H(1,7,6,60)=0.401243432
H(2,7,6,60)=0.122829622
H(3,7,6,60)=0.016377283
H(4,7,6,60)=0.00096337
H(5,7,6,60)=2.22316E-05
H(6,7,6,60)=1.39821E-07
and the sanity check is that that lot does add up to 1 ie there are no other posibilities.
Now given that you are starting then the muligan is no longer an option: so you divide all the other answers by (1-H(0,7,6,60)) and set the probability of not starting to zero.
thus with six basics your starting hand will have the following basic counts when you start.
0 basics: probability is ZERO (by definition)
1 basic : probability is 0.741072581
2 basics: probability is 0.226858954
3 basics: probability is 0.03024786
4 basics: probability is 0.001779286
and its miniscule for the 5 and 6 card cases much as before.
So with 6 basics you will muligan half the time giving your opponents a single card advantage on average. And when you do start 3/4 or the time you will just have a lone basic and 1/5 of the time you will be blessed with two basics. Once in every thirty games you will have three basics when you start.
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HTH :-D
[edit: SteveP's original assertion that we will see some viable decks with very few REAL basics may well be true. It does hinge on the number of draw cards in your opening hand. I might take EZ1's hint, provided that I can adequately explain the reasoning behind the numbers!
]