Logical Reasoning
MediumQ. No. find-the-next-number-in-the-series-51-45-39-33-27
Find the next number in the series: 51, 45, 39, 33, 27, ?
Last Updated:
A21
B27
C30
D24
Correct Answer: 21Your Answer: A
The sequence follows a logical number pattern.
FAQ
Common questions and clear answers for this topic.
Number Series questions present a sequence of numbers following a specific pattern, and the candidate must identify the pattern to find the missing or wrong number. Common types: Arithmetic series (constant difference), Geometric series (constant ratio), Square/cube series, Prime number series, Fibonacci series, and Alternating series. Number Series is frequently asked in SSC CGL, IBPS Bank PO, RRB NTPC, and SBI Clerk exams.
An Arithmetic Progression (AP) is a number series where the difference between consecutive terms is constant (common difference d). Example: 2, 5, 8, 11, 14 (d=3) or 20, 16, 12, 8 (d=-4). Formula: nth term = a + (n-1)d where a is first term. To identify AP, subtract consecutive terms - if the difference is constant, it is an AP. These are the easiest number series questions in competitive exams.
A Geometric Series is a number series where each term is obtained by multiplying the previous term by a constant ratio (r). Example: 2, 6, 18, 54, 162 (r=3) or 256, 128, 64, 32, 16 (r=0.5). Formula: nth term = a times r^(n-1). To identify geometric series, divide each term by the previous term - if the ratio is constant, it is geometric. Square and cube series are variations involving perfect squares or cubes.
Wrong Number Series questions give a sequence with one incorrect number. Strategy: 1) Look at each number relative to others, 2) Try to identify the pattern (AP, GP, squares, cubes, mixed operations), 3) Find which number breaks the pattern, 4) That is the wrong number. Practice multiple series patterns for accuracy. These questions appear in IBPS PO, SBI PO, and RBI Grade B exams.
Quick tips for Number Series: 1) Check differences first (AP pattern), 2) If differences increase, check second differences (quadratic), 3) Check ratios for geometric series, 4) Look for squares (1,4,9,16,25), cubes (1,8,27,64,125), primes (2,3,5,7,11,13), 5) Check alternate terms separately in mixed series, 6) For two-step series check if times n plus or minus constant, 7) Practice 20-30 questions daily.
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