Question:medium

Find the missing number in the series: 3, 7, 15, 31, 63, __?

Show Hint

In number series problems, always check for common patterns such as:
  • Multiplication followed by addition/subtraction
  • Squares or cubes of numbers
  • Powers of 2 or 3
  • Increasing differences between terms
For this series: \[ 3, 7, 15, 31, 63 \] The pattern is: \[ 2^2-1,\; 2^3-1,\; 2^4-1,\; 2^5-1,\; 2^6-1 \] So the next term is: \[ 2^7 - 1 = 127 \]
Updated On: Mar 16, 2026
  • 95
  • 111
  • 127
  • 135
Show Solution

The Correct Option is C

Solution and Explanation

This is a number series problem where we need to identify the pattern governing the sequence of numbers to find the next term.
Step 1: Understanding the Question:
The task is to find the number that logically follows 63 in the given series.
Step 2: Key Formula or Approach:
We can analyze the series in two common ways:
1. Find the relationship between consecutive terms.
2. Check if the terms follow a pattern related to powers of a number.
Step 3: Detailed Explanation:
Method 1: Relationship between consecutive terms
Let's see how each term is generated from the previous one.
\[ 3 \rightarrow 7: \quad (3 \times 2) + 1 = 6 + 1 = 7 \] \[ 7 \rightarrow 15: \quad (7 \times 2) + 1 = 14 + 1 = 15 \] \[ 15 \rightarrow 31: \quad (15 \times 2) + 1 = 30 + 1 = 31 \] \[ 31 \rightarrow 63: \quad (31 \times 2) + 1 = 62 + 1 = 63 \] The pattern is consistent: Next Term = (Previous Term × 2) + 1.
Applying this rule to find the missing number:
\[ (63 \times 2) + 1 = 126 + 1 = 127 \] Method 2: Pattern based on powers of 2
Let's examine each term in relation to powers of 2.
\[ 3 = 4 - 1 = 2^2 - 1 \] \[ 7 = 8 - 1 = 2^3 - 1 \] \[ 15 = 16 - 1 = 2^4 - 1 \] \[ 31 = 32 - 1 = 2^5 - 1 \] \[ 63 = 64 - 1 = 2^6 - 1 \] The pattern is: The \(n\)-th term is \(2^{n+1} - 1\). The last given term (63) is the 5th term in the sequence, which is \(2^{5+1} - 1 = 2^6 - 1\).
The next term (the 6th term) would be \(2^{6+1} - 1\):
\[ 2^7 - 1 = 128 - 1 = 127 \] Both methods yield the same result.
Step 4: Final Answer:
The missing number in the series is 127.
\[ \boxed{127} \]
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