Math Problem Statement

If a collection of pencils is placed in rows of 3, there are 2 pencils left; if placed in rows of 5, there are 2 left; and if placed in rows of 7, there are 2 left. What is the smallest possible number of pencils in the collection?

Solution

To solve this problem, we are looking for the smallest positive integer NN that satisfies the following conditions:

  1. N2(mod3)N \equiv 2 \pmod{3}
  2. N2(mod5)N \equiv 2 \pmod{5}
  3. N2(mod7)N \equiv 2 \pmod{7}

Each of these conditions states that if the pencils are divided into rows of 3, 5, or 7, there are always 2 pencils left over. This means N2N - 2 must be divisible by 3, 5, and 7.

Step 1: Rewrite the Problem

Since N2(mod3)N \equiv 2 \pmod{3}, N2(mod5)N \equiv 2 \pmod{5}, and N2(mod7)N \equiv 2 \pmod{7}, we can rewrite these congruences as: N=3k+2N = 3k + 2 N=5m+2N = 5m + 2 N=7n+2N = 7n + 2 for integers kk, mm, and nn.

Step 2: Reduce to a Common Congruence

Since N2N - 2 must be divisible by 3, 5, and 7, the smallest such number N2N - 2 that meets these criteria is the least common multiple (LCM) of 3, 5, and 7.

The LCM of 3, 5, and 7 is: [ \text{LCM}(3, 5

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Math Problem Analysis

Mathematical Concepts

Modular Arithmetic
Least Common Multiple (LCM)
Number Theory

Formulas

N ≡ 2 (mod 3)
N ≡ 2 (mod 5)
N ≡ 2 (mod 7)

Theorems

Chinese Remainder Theorem

Suitable Grade Level

Grades 8-10