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乘 1 000 反过来 = songsang darab 1 000 乘 1 000 会在后面补三个 0。5 000 去掉三个 0，剩下 5，所以 □ = 5。检查：5 × 1 000 = 5 000 ✔。 例子：□ × 100 = 700 → 7。 注意：50 × 1 000 = 50 000，太大了。

undo × 1 000 = songsang darab 1 000 Multiplying by 1 000 adds three zeros. Take three zeros off 5 000 and 5 is left, so □ = 5. Check: 5 × 1 000 = 5 000 ✔. Example: □ × 100 = 700 → 7. Watch out: 50 × 1 000 = 50 000, which is too big.

逐个试 = cuba satu demi satu 411 × 2 = 822（三位数 ✘）；511 × 2 = 1 022（四位数 ✔）。所以最小的数字是 5。只要百位乘 2 达到 10 个百，就会变成四位数。 例子：2 × 500 = 1 000。 注意：从小到大试，第一个符合的就是答案。

try one by one = cuba satu demi satu 411 × 2 = 822 (three digits ✘); 511 × 2 = 1 022 (four digits ✔). The smallest digit is 5. As soon as the hundreds times 2 reaches 10 hundreds, the product has four digits. Example: 2 × 500 = 1 000. Watch out: try from small to big; the first one that works is the answer.