100+ Series Completion Questions for NTSE with Answers - 1

Ans: C

Clearly, 3 + 6 = 9, 9 + 6 = 15,….

So, the series is an A.P. in which a = 3 and d = 6.

∴ 21st term = a + (21 - 1) d = a + 20d = 3 + 20 × 6 = 123.

Ans: C

Clearly, 5 + 3 = 8, 8 + 3 = 11, 11 + 3 = 14, …

So, the series is an A.P. in which a = 5 and d = 3.

Let 320 be the nth term of the series.

Then, 320 = 5 + (n - 1) × 3 or (n - 1) = 105 or n = 106.

Ans: B

The given series consists of numbers each of which, on dividing by 7, leaves a remainder 3. No other number except 346 satisfies the property.

Ans: C

Clearly, 5 × 2 = 10, 10 × 2 = 2, 20 × 2 = 40,….

So, the series is a G.P. in which a = 5 or r = 2. Let 1280 be the nth term of the series.

Then, 5 × 2^{n -1} = 1280

⇔ 2^{n – 1} = 256 = 2⁸

⇔n - 1 = 8

⇔ n = 9.

Ans: A

The given number consists of cubes of natural numbers only. 256 is not the cube of any natural number.