if the pattern below follows the rule "Starting with nine, every consecutive line has a number one less than the previous line," how many marble must be in the seventh line

3

Step-by-step explanation:

Given that there is a pattern in the placing of marbles on the line

9 marbles on I line

8 on second, 7 on third etc.

We find that this follows an arithmetic sequence with I term = 9 and common difference = - 1

Hence marbles on 7th line

=value of 7th term in arithmetic sequence

=a + (n-1) d

Here a = I term = 9

n = line to be determined = 7

d = - 1

Substitute to get

9 + (7-1) (-1)

=3

Hence 3 marbles is the answer.