偵探狗汪探長要打開一個個保險箱，密碼都是大數。孩子把數字拖進「千萬、百萬、十萬、萬｜千、百、十、個」八格密碼盤：第 1 關照中文讀法寫出數字，練習最容易錯的「中間有 0」；第 2 關做位值換算（例如 3 個百萬和 25 個萬合起來），再看「多 1 個萬」時 9 一格一格翻成 0、往前進位；第 3 關照線索一位一位推出密碼。
適合年級：四年級（約 9～10 歲）。對應 108 課綱（十二年國教）數學單元：四年級「一億以內的數（萬到千萬的位值）」。各版本教科書的單元順序不同，完整對照請看國小數學課綱對照表。免費、無廣告、不用註冊，手機、平板、電腦打開就能玩，題目有注音，也會念出密碼。
陪玩小提醒：大數四位一節，先讀「萬」那一節，再讀後面四位；中間連續好幾個 0 只讀一個「零」，但寫的時候每一個 0 都要寫出來。
Detective Woof has safes to open, and every code is a big number. Children drag digits into eight slots from ten millions to ones: level 1 writes the number from how it is read (zeros in the middle are the tricky part), level 2 builds numbers from place-value parts such as 3 millions and 25 ten thousands and watches carries roll from slot to slot, and level 3 works out the code from clues.
Suits Grade 4 (about age 9–10) in Taiwan. Taiwan curriculum (12-year basic education) unit: Numbers to 100 million (Grade 4). Free with no ads and no sign-up, in Traditional Chinese with zhuyin and in English, with codes read aloud. See the Taiwan elementary maths curriculum map.