Answer:
d=$15,
Explicit formula:
At 27 month Ginni make a deposit at least $500.
Step-by-step explanation:
Given that the deposits Ginny makes at her bank each month form an arithmetic sequence. The deposit for month 3 is $150, and the deposit for month 5 is $180.
we have to find the common difference i.e d
-- , -- , $150, x, $180 , -------
Let $x be the 4th deposit.
∴ x-150=180-x ⇒ 2x=330 ⇒ x=165
Common difference,d=165-150=$15
Let us find the first tem i.e the value of a
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Explicit formula for arithmetic sequence is
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Now, we have to find the amount of Ginni in 12th deposit i.e n=12
⇒
Now, we have to find at what month Ginni make a deposit at least $500.
⇒
⇒
⇒
hence, at 27 month Ginni make a deposit at least $500.