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Subjects · Leaving Cert Computer Science

Leaving Cert Computer Science: Problem solving & puzzles

How often Problem solving & puzzles comes up on the Computer Science papers, every year it was asked, and questions to try.

HL Asked on 5 of the last 5 Higher Level papers, most recently in 2025. banker

OL Asked on 2 of the last 5 Ordinary Level papers, most recently in 2023.

Problem solving & puzzles, Higher Level(8 marks)

Quick ones on Problem solving & puzzles.

(a)In the fox, goose and grain river puzzle, what must the farmer take across first?
  1. The fox
  2. The goose
  3. The grain
(b)9 coins look alike; one is lighter. Fewest balance weighings that always find it?
  1. 4
  2. 3
  3. 2
(c)What is the minimum number of moves to solve the Tower of Hanoi with 3 discs?
  1. 7
  2. 6
  3. 8
Show the answers

(a) The goose

(b) 2

(c) 7

Your turn: Higher Level questions on Problem solving & puzzles.

Higher Level

Asked on 5 of the last 5 Higher Level papers, most recently in 2025. banker

Every paper, year by year

YearWhere it came up
2025A3
2024A4
2023A8, A9
2022A10
2021B14

Links open the State Examinations Commission’s paper for that year.

More Problem solving & puzzles questions

Problem solving & puzzles, 3 marks

A snail climbs 3 m up a 10 m well each day and slips back 2 m each night. On which day does it reach the top?

  1. Day 10
  2. Day 7
  3. Day 8
Show the answer

Day 8

It gains 1 m per full day, so it starts day 8 at 7 m. That day it climbs 3 m and reaches 10 m before it can slip. Counting 1 m a day gives the wrong answer 10.

Problem solving & puzzles, 3 marks

A drawer has only black and white socks. How many must you take out, without looking, to be sure of a pair?

  1. 4
  2. 3
  3. 2
Show the answer

3

With 2 socks you could have one of each colour. There are only 2 colours, so the 3rd sock must match one of the first two.

Problem solving & puzzles, 2 marks

What is trial and error as a problem-solving strategy?

  1. Trying a solution, checking it and adjusting until it works
  2. Taking the first answer found and not checking it any further
  3. Solving the problem by working backwards from the goal
Show the answer

Trying a solution, checking it and adjusting until it works

Trial and error suits problems with few possible answers or no known method. Each failed attempt gives information for the next try.

Ordinary Level

Asked on 2 of the last 5 Ordinary Level papers, most recently in 2023.

Every paper, year by year

YearWhere it came up
2025Not asked
2024Not asked
2023B13
2022A12
2021Not asked

Links open the State Examinations Commission’s paper for that year.

More Problem solving & puzzles questions

Problem solving & puzzles, 3 marks

Tower of Hanoi with 3 discs: fewest moves needed to move the whole tower?

  1. 6
  2. 9
  3. 7
Show the answer

7

The fewest moves for n discs is 2 to the power n, minus 1. For 3 discs: 8 - 1 = 7. Each extra disc roughly doubles the moves.

Problem solving & puzzles, 2 marks

Trying every possible answer until one works is called…

  1. Decomposition
  2. Brute force
  3. Abstraction
Show the answer

Brute force

Brute force checks every option, e.g. trying all PINs from 0000 to 9999. It always finds the answer in the end but can be very slow.

Problem solving & puzzles, 3 marks

How many different 3-digit PINs can be made using the digits 0 to 9?

  1. 1000
  2. 30
  3. 720
Show the answer

1000

Each of the 3 places has 10 choices: 10 × 10 × 10 = 1000 PINs (000 to 999). Adding one more digit makes it 10 times harder to guess.

Other Computer Science topics

All of Leaving Cert Computer Science