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2 years ago

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A series contains 18 numbers and has a sum of 4,185. The last number of the series is 275. What is the first number?

1 answer:

dusya [7]2 years ago

3 0

The sum of arithmetic series is given by:
Sn=n/2(a1+an)
where:
n=number of terms
a1=first term
an=nth term
but
n=18, an=275, Sn=4185
plugging the values in the formula we get:
4185=18/2(a1+275)
simplifying this we get:
4185=9(a1+275)
dividing through by 9 we get:
465=a1+275
thus
a1=465-275
a1=190

Answer: first number is 190

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