Vedadots
Q4·CSAT · Prelims 2023

Maximum attempts to draw a red pair of shoes

NumericalPermutations & Combinations (Pigeonhole)Number theoryHard

Question

Raj has ten pairs of red, nine pairs of white and eight pairs of black shoes in a box. If he randomly picks shoes one by one (without replacement) from the box to get a red pair of shoes to wear, what is the maximum number of attempts he has to make ?

Options

a

27

b

36

c

44

d

45

Answer

Explanation

To determine the maximum number of attempts (the absolute worst-case scenario), we must assume Raj draws every single non-red shoe, and then draws single mismatched red shoes before completing a pair .

Convert shoe pairs into individual units: Red shoes = 10 × 2 = 20 units . White shoes = 9 × 2 = 18 units . Black shoes = 8 × 2 = 16 units.

Step 1: Draw all non-red shoes first to maximize failure: Total non-red = 18 (white) + 16 (black) = 34 attempts.

Step 2: Draw red shoes. To delay forming a pair, Raj could draw 10 shoes that all belong to the same foot orientation (e.g., 10 left red shoes) . Attempts added = 10.

Step 3: The very next shoe drawn (34 + 10 + 1 = 45th attempt) is guaranteed to be a right red shoe, completing a matching functional pair . Total maximum attempts = 34 + 10 + 1 = 45.

In worst-case selection puzzles involving pairs of objects, you must exhaust every single undesired item, select exactly one side of each desired item group, and then add 1 to guarantee a match.

Answer: (d).

Question details

Year

2023

Paper

CSAT

Question

Q4

Section

Numerical Ability

Sub-topic

Permutations & Combinations (Pigeonhole)

Type

Number theory

Difficulty

Hard

Source hint

Number theory

See all questions on Permutations & Combinations (Pigeonhole)

Browse every tagged question across all years

Explore →