# Which term of the AP : 3, 8, 13, 18, . . . ,is 78?

Given,

First term, a = 3,

Common difference, d = a2 – a1 = 8 – 3 = 5,

nth term,  an = 78

To find: n = ?

We know that nth term of an A.P is given by:

an = a + (n – 1)d

78 = 3 + (n – 1)5

(n – 1) 5 = 78 – 3 = 75

5n - 5 = 75

5n = 80

n = 80/5

n = 16

Thus, 78 is the 16th term of given AP.

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