A bag contains the numbers 1 through 20 inclusive. If three are selected at random and placed in order from smallest to largest, find the probability that the arrangement is of three consecutive integers.
Here is another way.
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Probability is favorable to possible.
What is favorable?
{1,2,3} All premutations of this set = 6
{2,3,4} All premutations of this set = 6
.....
{18,19,20} All premutations of this set = 6
In total we have,
18 Sets
Thus,
The possible outcomes is the number of ways of selecting 3 out of twenty.
Which is,
Thus,