FREE AIME Number Theory Questions and Answers

0%

How many positive divisors does the number 360 have?

正しい! 間違い!

The prime factorization of 360 is 22 ⋅ 32 ⋅ 5 The number of divisors is (3+1)(2+1)(1+1)=4×3×2=24.

What is the greatest common divisor (GCD) of 252 and 198?

正しい! 間違い!

To find the GCD, we can use the Euclidean algorithm. We divide 252 by 198, then 198 by the remainder, and repeat until the remainder is 0. The GCD is the last non-zero remainder.

Find the least positive integer 𝑛 such that 𝑛 is congruent to 1 modulo 4, 2 modulo 5, and 3 modulo 6.

正しい! 間違い!

We need to solve the system of congruences. Let 𝑛 = 4k+1. Substituting into the second congruence, 4k+1≡2 (mod5), we get 4𝑘 ≡ 1 (mod5). Since 4 and 5 are coprime, we find 𝑘 (mod5), so 𝑘 = 5m+4.
Substituting into the third congruence, 𝑛 = 4(5m+4)+1=20m+17, we solve 20m+17 ≡ 3(mod6), giving 20m ≡ −14 ≡ 4(mod6). Thus 2m ≡ 2(mod3), so 𝑚 ≡ 1 (mod3). The smallest positive 𝑚 is 1, so 𝑘 = 5(3p+1)+4. Substituting back, we get 𝑛 = 20×3×p+17, and the smallest positive 𝑛 is 58.

What is the smallest positive integer 𝑥 such that 𝑥 ≡ 2 (mod 3) and 𝑥 ≡ 3 (mod5)?

正しい! 間違い!

To find 𝑥 we solve the system of congruences. Starting with x=3k+2, we substitute into the second congruence 3k+2 ≡ 3(mod5), giving 3𝑘 ≡ 1(mod5).
Solving for 𝑘, we find 𝑘 ≡ 2(mod5), so 𝑘 = 5m+2. Substituting back, 𝑥 = 3(5m+2)+2=15m+8, and the smallest positive 𝑥 is 8.

If 𝑎 and 𝑏 are relatively prime, which of the following statements is true?

正しい! 間違い!

Relatively prime numbers have a GCD of 1, meaning they share no common factors other than 1.