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

First term = a = 3,

Common difference = d = 8 – 3 = 13 – 8 = 5

$$a_n$$ = 78

Using formula $$a_n = a + (n - 1)d$$, to find $$n^{th}$$ term of arithmetic progression,

$$\Rightarrow$$ 78 = 3 + (n - 1) 5
$$\Rightarrow$$ 75 = 5n - 5
$$\Rightarrow$$ 80 = 5n
$$\Rightarrow$$ n = 16

It means $$16^{th}$$ term of the given AP is equal to 78.