熱氣球只能降落在整十、整百、整千、整萬的停機坪，孩子拖著熱氣球飛到「這個數取概數後」的位置：先在數線上看到最近的整百，再練大數取概數，最後用概數估算兩段航程，並判斷什麼時候要無條件進入（座位要夠）、什麼時候要無條件捨去（綁滿的才算）。
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「概數（四捨五入）和估算」。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，也會念出算式。
陪玩小提醒：四捨五入看「要取的那一位的下一位」；生活中不一定都用四捨五入，像坐車要用無條件進入，裝箱販賣要用無條件捨去。
Balloons can only land on pads at round tens, hundreds, thousands and ten thousands, so children fly each balloon to where its number rounds to: first with the number marked on the line, then with big numbers, and finally estimating two flights by rounding, and choosing when to round up (enough seats) or down (only full bunches count).
Suits Grade 4 (about age 9–10) in Taiwan. Taiwan curriculum (12-year basic education) unit: Rounding and estimating (Grade 4). Free with no ads and no sign-up, in Traditional Chinese with zhuyin and in English, with questions read aloud. See the Taiwan elementary maths curriculum map.