Math Problem Statement

A student has a penny collection. When the student puts the Pennie’s in groups of 2 there is one penny left over. When they are put in groups of three, four, five, and six there is also one penny left over. When they are put in groups of seven there are not Pennie’s left over. What is the smallest number of pennies the student could have?

Solution

We need to find the smallest number of pennies xx that satisfies the following conditions:

  1. When divided by 2, there is 1 penny left over.
    x1(mod2)x \equiv 1 \pmod{2}

  2. When divided by 3, there is 1 penny left over.
    x1(mod3)x \equiv 1 \pmod{3}

  3. When divided by 4, there is 1 penny left over.
    x1(mod4)x \equiv 1 \pmod{4}

  4. When divided by 5, there is 1 penny left over.
    x1(mod5)x \equiv 1 \pmod{5}

  5. When divided by 6, there is 1 penny left over.
    x1(mod6)x \equiv 1 \pmod{6}

  6. When divided by 7, there is no remainder.
    x0(mod7)x \equiv 0 \pmod{7}

Approach:

Since the first five conditions imply x1(mod2,3,4,5,6)x \equiv 1 \pmod{2, 3, 4, 5, 6}, the number x1x-1 must be divisible by the least common multiple (LCM) of 2, 3, 4, 5, and 6. The LCM of these numbers is 60.

So, x1x-1 must be a multiple of 60, meaning: x=60k+1x = 60k + 1 for some integer kk. Additionally, xx must also be divisible by 7: 60k+10(mod7)60k + 1 \equiv 0 \pmod{7} 60k1(mod7)60k \equiv -1 \pmod{7} Since 604(mod7)60 \equiv 4 \pmod{7}, this simplifies to: 4k1(mod7)4k \equiv -1 \pmod{7} Multiplying both sides by the modular inverse of 4 modulo 7 (which is 2), we get: 8k2(mod7)8k \equiv -2 \pmod{7} k5(mod7)k \equiv 5 \pmod{7}

Thus, k=7m+5k = 7m + 5 for some integer mm, and substituting into x=60k+1x = 60k + 1: x=60(7m+5)+1=420m+301x = 60(7m + 5) + 1 = 420m + 301

The smallest solution is when m=0m = 0, which gives: x=301x = 301

Thus, the smallest number of pennies the student could have is 301.

Would you like further details or have any questions? Here are some follow-up questions for deeper understanding:

  1. How is the least common multiple (LCM) of several numbers calculated?
  2. What is the significance of modular arithmetic in problem-solving?
  3. How can we find the modular inverse of a number?
  4. What other problems can be solved using the Chinese Remainder Theorem?
  5. Can this problem be solved using a different method?

Tip: When solving problems involving multiple remainders, the Chinese Remainder Theorem is often a helpful tool to find solutions systematically.

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Math Problem Analysis

Mathematical Concepts

Modular Arithmetic
Least Common Multiple (LCM)
Chinese Remainder Theorem

Formulas

x ≡ 1 (mod 2)
x ≡ 1 (mod 3)
x ≡ 1 (mod 4)
x ≡ 1 (mod 5)
x ≡ 1 (mod 6)
x ≡ 0 (mod 7)

Theorems

Chinese Remainder Theorem

Suitable Grade Level

Grades 9-12