Four dice are rolled. The number of ways in which at least one die shows 3, is
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Here is a way by PIE.
Number of ways you can have one die show up 3 is ( 1 4 ) × 6 3 because you have 4 options for which die is 3 and each of the other 3 die have 6 possible numbers. Similarly, the number of ways two dice show up 3 is ( 2 4 ) × 6 2 , three dice is ( 3 4 ) × 6 , and four dice is just 1 .
The number of ways for two dice is counted ( 1 2 ) = 2 times, so subtract it once. The number of ways for three dice is counted ( 1 3 ) − ( 2 3 ) = 0 times, so add it once. The number of ways for four dice is counted ( 1 4 ) − ( 2 4 ) + ( 3 4 ) = 2 times, so subtract it once.
Total: ( 1 4 ) 6 3 − ( 2 4 ) 6 2 + ( 3 4 ) 6 − ( 4 4 ) = 6 7 1
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The number of possible outcomes is 6 4 because there are 6 possible numbers on each die and there are 4 dice so 6 x 6 x 6 x 6 possible outcomes. The number of outcomes without a three on any of the dice is 5 4 because there are five possible outcomes for each die if 3 is not on any of them. So subtracting this from the total number of outcomes leaves 6 4 − 5 4 = 6 7 1 outcomes where a three is on at least one of the dice.