as I said, FAPP (for (almost) all practical purposes) in MTG you don't need statistics, with the exception of combo-like decks (but you can get the feeling for statistics either by playing the deck thousands of times or just calculating the probabilities)

all good magic players have an intuitive notion of what is good an what is bad - clearly everybody will tell you that 12 Islands is too little for BBS and 21 Islands is overkill. but the fine tuning is more subtle - is 17 Islands+1 Strip+1 LoA better than 19 Islands? A tough decision and harder to calculate/estimate.

the math is simple: if you have a deck of hundred cards and you draw one card and each card in your deck is different you have a chance of 1% to draw a given card (I hope that's simple enough). i.e. the chance of NOT drawing this card is 99% (or 0.99). if you draw a second card without putting the first card back the chance of NOT drawing the marked card within the two cards is the chance of NOT drawing the marked card in the first draw times NOT drawing the card in the second draw, i.e. 99*98/100*99=98/100=0.98=98% (because when you draw the second card you have only 99 cards left, 98 of which are not the marked one). This means, the chance to actually DRAW the marked card is 1-99*98/100*99=2% (wow, what a miracle!).

Now of course, this simple example is too simple, since it involves only 1 marked card. Lets proceed to 2 marked cards: For the first draw you have
2% probability of drawing one of the 2 marked cards. For the second draw you have the probability 98*97/100*99 of NOT drawing one of the 2 cards, i.e. 1-98*97/100*99=3.979...%, i.e. the chances of drawing at least one of the two marked cards within the first 2 draws is LESS than 4%.

Adjust the decksize and the number accordingly and you obtain the formula I have posted.

BTW, I rarely use such calculations for deckbuilding - but sometimes it pays (especially if you start from scratch)