Answer
2187
Work Step by Step
See page 690, The Fundamental Counting Principle:
The number of ways in which a series of successive things can occur is found by multiplying the number of ways in which each thing can occur.
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Ways in which the
1st stock can change: 3
2nd stock can change: 3
...
6th stock can change: 3
7th stock can change: 3
Total: $3\times 3\times...\times 3=3^{7}=2187$
