Foundation
Mental Ability Foundation
Series Completion Subtopic 1
Question

Nitin was counting down from 32. Sumit was counting upwards, the numbers starting from 1 and he was calling out only the odd numbers. What common number will they call out at the same time if they were calling out at the same speed ?

They will not call out the same number

21

22

19

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Solution

Understanding the Problem

Nitin counts down from 32: 32, 31, 30, ..., 1.

Sumit counts up odd numbers from 1: 1, 3, 5, 7, ...

They call numbers at the same speed (same time intervals), so we need to find a number that appears in both sequences at the same position (step).

Step 1: Define the Sequences

Let n be the step number (starting from 1).

Nitin's number at step n: an=32-n+1=33-n

Sumit's number at step n: bn=2n-1 (the nth odd number)

Step 2: Set the Numbers Equal

We need an=bn:

33-n=2n-1

Step 3: Solve for n

33+1=2n+n

34=3n

n=34311.333

Since n must be an integer (step number), there is no integer solution. This suggests they never call the same number at the same step.

Step 4: Verify with Integer Steps

Let's check a few steps around n=11:

  • At n=11: Nitin: 33-11=22, Sumit: 2*11-1=21 → Not equal
  • At n=12: Nitin: 33-12=21, Sumit: 2*12-1=23 → Not equal
  • At n=10: Nitin: 33-10=23, Sumit: 2*10-1=19 → Not equal

No match found. They never call the same number simultaneously.

Final Answer

They will not call out the same number.

Related Topics

  • Arithmetic Sequences: A sequence where each term is obtained by adding a constant difference to the previous term.
  • Linear Equations: Equations of the first degree (e.g., ax + b = c).
  • Integer Solutions: Solutions to equations that must be integers; often arise in discrete counting problems.

Key Formulae

  • nth term of a decreasing arithmetic sequence: an=a+(n-1)d, where d is negative. For Nitin: first term=32, d=-1, so an=32-(n-1)=33-n.
  • nth odd number: 2n-1.