Question

Andy stacked a pile of wooden logs. The bottom row contained 30 logs. The rows above it held 27 logs, 24 logs and so on. Determine the number of logs in the 9th row.

A- 3 Logs

B- 6 Logs

C- 9 Logs

D- 10 Logs

Answer 1

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.