Using the permutation formula, it is found that there are 840 ways to place the letters.
The order in which the letters are placed is important, as PAST is a different arrangement that PTAS, for example, hence the permutation formula is used.
The number of possible permutations of x elements from a set of n elements is given by:
[tex]P_{(n,x)} = \frac{n!}{(n-x)!}[/tex]
In this problem, 4 letters are taken from a set of 7, hence:
[tex]P_{7,4} = \frac{7!}{3!} = 840[/tex]
There are 840 ways to place the letters.
More can be learned about the permutation formula at https://brainly.com/question/25925367
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