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Q7·CSAT · Prelims 2023

Remainder of product divided by 100

NumericalRemaindersNumber theoryMedium

Question

What is the remainder when 85 × 87 × 89 × 91 × 95 × 96 is divided by 100?

Options

a

0

Answer
b

1

c

2

d

4

Explanation

To check for a remainder of 0 modulo 100, analyze the prime factors 2 and 5 within the continuous product expression : 85 = 5 × 17 \implies contains one factor of 5. 95 = 5 × 19 \implies contains one factor of 5. Total factors of 5 from these two odd numbers = 5 × 5 = 25.

Now scan for even terms: 96 = 32 × 3 = 2^5 × 3 \implies contains multiple factors of 2, specifically a multiple of 4.

Multiplying our extracted factors yields: ⟨MATH⟩25 × 4 = 100⟨/MATH⟩. Because the product contains 100 as an explicit factor, the entire expression is perfectly divisible by 100, leaving a remainder of exactly 0.

Before executing complex modular division, check if the numbers across the expression supply enough standalone factors of 2 and 5 to assemble the base denominator.

Answer: (a).

Question details

Year

2023

Paper

CSAT

Question

Q7

Section

Numerical Ability

Sub-topic

Remainders

Type

Number theory

Difficulty

Medium

Source hint

Number theory

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