猴子妈妈采来了一篮桃子,她让小猴子数一数共采了多少桃子。小猴子3个3个地数,最后多出一个,它就把多出...

Создано: 8 февраля 2025 г.

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Вопрос

猴子妈妈采来了一篮桃子,她让小猴子数一数共采了多少桃子。小猴子3个3个地数,最后多出一个,它就把多出的一个扔在一边;它又5个5个地数,到最后还是多出一个,它又把多出的一个扔在一边;最后它7个7个地数,还是多出一个。它数了三次,到底有多少桃子,还是不清楚。小朋友,你知道这篮子里至少有多少桃子吗?

Мысль

Figuring out the peach count

OK, the user is presenting a math problem where the peach count involves remainders. The monkey counts in groups of 3, 5, and 7, always with a remainder of 1. This suggests we're dealing with a number that leaves a remainder of 1 when divided by 3, 5, and 7.

To solve this, I need to find the least common multiple (LCM) of 3, 5, and 7, then add 1. The smallest number that meets these conditions is 106.

Checking the peach count

The monkey discards extra peaches each time, but the problem is asking for the smallest number of peaches where N mod 3 = 1, N mod 5 = 1, and N mod 7 = 1. This means N-1 is divisible by 3, 5, and 7, so N-1 must be the LCM of those numbers, which is 105. Thus, N = 106.

But hold on, what about k=0? If k=0, that would suggest N=1, which doesn't make sense since the problem specifies a basket of peaches. So 106 is indeed the correct answer.

Confirming the peach count

The problem suggests a number that leaves a remainder of 1 when divided by 3, 5, and 7. So, N-1 must be divisible by those numbers, and N-1 = 105k. The smallest solution is when k=1, which gives N=106. However, N=1 could technically work with remainders, but that doesn't align with the context of "a basket of peaches," indicating there must be at least a few peaches. Checking N=106 confirms it satisfies all the conditions, making 106 the correct answer.

Reasoned for 9 seconds

Ответ

设桃子的总数为 NN。题目给出的条件可以转化为以下同余条件:

{N1(mod3),N1(mod5),N1(mod7).\begin{cases} N \equiv 1 \pmod{3},\\[1mm] N \equiv 1 \pmod{5},\\[1mm] N \equiv 1 \pmod{7}. \end{cases}

这说明 N1N-1 同时被 3、5、7 整除,即

N1=k×lcm(3,5,7),N - 1 = k \times \text{lcm}(3,5,7),

其中 kk 为正整数。由于 3、5、7 互质,所以它们的最小公倍数是

lcm(3,5,7)=3×5×7=105.\text{lcm}(3,5,7) = 3 \times 5 \times 7 = 105.

因此,

N1=105kN=105k+1.N - 1 = 105k \quad \Rightarrow \quad N = 105k + 1.

题目要求桃子至少有多少个,即取最小的正整数 k=1k=1 得到:

N=105×1+1=106.N = 105 \times 1 + 1 = 106.

验证一下:

  • 106÷3=35106 \div 3 = 3511
  • 106÷5=21106 \div 5 = 2111
  • 106÷7=15106 \div 7 = 1511

因此,这篮桃子至少有 106 个。

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