[Math] Four cards are drawn without replacement. What is the probability of drawing at least two kings

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Four cards are drawn without replacement. What is the probability of drawing at least two kings?

is the below my answer correct or not?!

since cards are drawn without replacement, (4/52)(3/51)(48/50)*(47/49)=1/240=0.0042

1-0.0042=0.9958

Probability of drawing AT LEAST 2 kings is 0.9958.

Best Answer

Consider a tree diagram:enter image description here

Traveling along a green line segment will correspond in this picture to having drawn a king, and along a red meaning you drew something other than a king.

To arrive at a particular leaf occurs with a probability equal to the multiplication of the probabilities of each branch leading to it.

It would be quicker in this case to calculate all of the probabilities for leafs which are not labeled blue (corresponding to having not drawn 2 or more kings) and subtracting that probability from 1.

So then, it is $$1 - (4\cdot\dfrac{4\cdot 48\cdot 47\cdot 46}{52\cdot51\cdot50\cdot49}+\dfrac{48\cdot47\cdot46\cdot45}{52\cdot51\cdot50\cdot49}) \approx .0257$$

Equivalently, you could consider it via counting principles and inclusion-exclusion. The probability of ending with only one king would be the total number of ways of ending with 1 king divided by the total number of ways to draw 4 cards. The total number of ways of ending with 1 king can be broken into steps: a) pick the spot for the king (4 ways), b) pick which king (4 ways), c) pick the first not-king (48 ways), d) pick the second not-king (47 ways), e) pick the third not-king (46 ways). Divide by the total number of ways of drawing 4 cards (52*51*50*49). This number is in fact the first fraction in the above formula.

Do so similarly for counting for the case of no kings drawn.