Input Output Reasoning Questions for Bank and SSC Exams
Find what moved between two steps, state the rule in one line, then apply it.
In input output reasoning, a machine takes a line of words or numbers and changes it one step at a time. You are shown the input and a few steps, you find the rule, and then you apply it to a new input.
The rule is mechanical. Once you can say in one sentence what moved, or what changed, between two steps, every question in the set becomes routine.
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Types of Input-Output Questions
| Type | What the machine does |
|---|---|
| Number sorting | Arranges numbers in ascending or descending order, one number per step |
| Word sorting | Arranges words in dictionary order, one word per step |
| Alternating sorting | Moves a word or number to the left end in one step and to the right end in the next |
| Mixed words and numbers | Sorts words and numbers separately in the same line |
| Arithmetic change | Applies the same calculation to every number at each step |
How to Find the Rule
- 1
Compare the input with Step I
Find what moved and where. If only values changed, it is an arithmetic machine.
- 2
Ask why that element moved
Was it the smallest, the largest, or first or last in dictionary order?
- 3
Confirm with Step II
The same rule must explain the next move. If Step II moves to the other end, the machine alternates.
- 4
Note how the rest shift
In most sorting machines, the elements not yet arranged keep their order and simply slide along.
- 5
Know when it stops
The machine stops when the line is fully arranged. If the remaining elements are already in order, the last step comes early.
Practice question 1: Number Sorting
Rule: in each step, the smallest number not yet arranged moves to the left, just after the numbers already arranged. The others keep their order. Input: 48 15 72 33 91 26. What is Step II, and how many steps does the machine take?
- 1
Step I
The smallest number is 15. It moves to the left end: 15 48 72 33 91 26.
- 2
Step II
The next smallest is 26. It moves to sit after 15: 15 26 48 72 33 91.
- 3
Step III
The next smallest is 33. It moves to sit after 26: 15 26 33 48 72 91.
- 4
Check for the end
48, 72 and 91 are already in ascending order, so the line is fully arranged.
- 5
Answer
Step II is 15 26 48 72 33 91, and the machine stops at Step III.
Practice question 2: Alternating Word Sorting
Rule: in odd-numbered steps, the word that comes first in dictionary order (among those not yet arranged) moves to the left, just after any words already placed on the left. In even-numbered steps, the word that comes last in dictionary order moves to the right, just before any words already placed on the right. Input: tree bank lamp gold rose. Which step reads "bank lamp gold rose tree"?
- 1
Dictionary order
bank, gold, lamp, rose, tree.
- 2
Step I
First word: bank moves to the left end. bank tree lamp gold rose.
- 3
Step II
Last word: tree moves to the right end. bank lamp gold rose tree.
- 4
Step III
Next first word: gold moves to sit after bank. bank gold lamp rose tree. The line is now in dictionary order, so the machine stops.
- 5
Answer
"bank lamp gold rose tree" is Step II.
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Practice question 3: Arithmetic Machine
Rule: at every step, each even number is divided by 2 and each odd number is multiplied by 2. Input: 12 7 20 9. What is the sum of the numbers in Step II?
- 1
Step I
12 ÷ 2 = 6, 7 × 2 = 14, 20 ÷ 2 = 10, 9 × 2 = 18. Step I: 6 14 10 18.
- 2
Step II
All four are even, so each is halved: 3 7 5 9.
- 3
Add
3 + 7 + 5 + 9 = 24.
- 4
Answer
The sum in Step II is 24.
Practice question 4: Starting From a Given Step
Using the rule from Practice question 1, Step II of a new input is: 11 18 64 29 50 37. What is the last step, and which step is it?
- 1
Read what is done
Step II means two numbers are arranged: 11 and 18 sit at the left in order.
- 2
Step III
Next smallest is 29: 11 18 29 64 50 37.
- 3
Step IV
Next smallest is 37: 11 18 29 37 64 50.
- 4
Step V
Next smallest is 50: 11 18 29 37 50 64. Fully arranged.
- 5
Answer
The last step is Step V: 11 18 29 37 50 64.
You Cannot Always Go Backwards
If a question gives Step II and asks for the input, check whether the input can be found at all. In Practice question 4, 11 and 18 could have started in many positions, so the input cannot be determined.
Common Traps
- check_circleAssuming the machine always runs to a fixed number of steps. It stops as soon as the line is arranged.
- check_circleShifting the other elements in the wrong order. In most machines they keep their original order.
- check_circleIgnoring ties in word length or value. Read whether the question breaks ties by dictionary order or by position.
- check_circleGuessing the input from a later step. Work forwards; go backwards only if every position can be fixed.
How Bank and SSC Papers Test Input-Output
The IBPS PO syllabus lists input-output machines under reasoning, tested in Reasoning Ability in prelims (35 questions, 40 marks) and in Reasoning in mains (40 questions, 60 marks). It usually comes as one input with its steps, followed by several questions on a new input: the value at a given step, the number of steps, or the position of an element.
The SSC CGL reasoning topic list does not name input-output, so treat it as a bank exam topic.
FAQs
How do I find the rule in input output reasoning?expand_more
Compare the input with Step I and note which element moved and where. Confirm the rule with Step II before using it.
How do I know which step is the last one?expand_more
The machine stops when the whole line is arranged. Count only the steps that move something; if the rest is already in order, the last step comes early.
Can the input be found from a later step?expand_more
Sometimes not. If several starting positions would produce the same step, the input cannot be determined, and that option is the correct answer.
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