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Which term of the AP: 3, 8, 13, 18, …, is 78?
Given:
Given A.P. is $3, 8, 13, 18 ……$
To do:
We have to find $78$ is which term of the given A.P.
Solution:
Let $78$ be the nth term of the given A.P.
Here,
$a_1=3, a_2=8, a_3=13$
Common difference $d=a_2-a_1=8-3=5$
We know that,
nth term $a_n=a+(n-1)d$
Therefore,
$a_{n}=3+(n-1)(5)$
$78=3+n(5)-1(5)$
$78-3=5n-5$
$75+5=5n$
$5n=80$
$n=\frac{80}{5}$
$n=16$
Hence, $78$ is the 16th term of the given A.P.  
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