Answer:
B. 6
Explanation:
You want the 9th term of the arithmetic sequence with first term 30 and common difference -3.
N-th term
The n-th term of an arithmetic sequence is given by ...
an = a1 -d(n -1 . . . . . . . first term a1, common difference d
Application
For the given first term and difference, the formula for the n-th term is ...
an = 30 -3(n -1)
The 9th term is ...
a9 = 30 -3(9 -1) = 30 -24
a9 = 6
There are 6 logs in the 9th row.
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Additional comment
Sometimes, the answer can be found quicker by simply listing the terms:
30 27 24 21 18 15 12 9 6
The 9th row has 6 logs.