鴛鴦管理員要看好水庫的水位。水庫旁邊有一把伸縮的百分尺，0% 固定在底部，孩子把尺的頂端拉到滿水位，讓 100% 對齊「全部」（每一題水庫的大小都不一樣）；左邊是水量的刻度，兩邊合起來就是課本裡的「雙數線」。第 1 關把水位拖到指定的百分率，或從水量讀出是百分之幾；第 2 關題目寫成「7 成」、「0.45」、「3/4」，要換成百分率，也要從百分率讀出有幾萬噸的水；第 3 關到遊客中心，同一把尺對齊門票的原價，打 8 折就拖到 80%，再算出要付多少元。
適合年級：五年級（約 10～11 歲）。對應 108 課綱（十二年國教）數學單元：五年級「百分率、折和成」。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，也會念出來。
陪玩小提醒：百分率是「每 100 份裡占幾份」，不管全部是 200 萬噸還是 2000 萬噸，100% 都是全部。1 成就是 10%；打 8 折是付 80%，也就是便宜 2 成。
Mandarin Duck looks after the reservoir. Beside it stands a stretchy percent ruler with 0% fixed at the bottom; children pull its top up to the full mark so that 100% lines up with the whole (each reservoir is a different size). The amount scale on the left and the ruler together make the double number line used in Taiwanese textbooks. Level 1 sets the water to a percentage or reads a percentage from an amount; level 2 gives “7 tenths (7成)”, “0.45” or “3/4” to turn into percentages, and reads amounts from percentages; level 3 moves to the visitor centre, where the same ruler lines up with a ticket price and “8折” means dragging to 80% and working out what to pay.
Suits Grade 5 (about age 10–11) in Taiwan. Taiwan curriculum (12-year basic education) unit: Percentages, discounts and tenths of a whole (Grade 5). Free with no ads and no sign-up, in Traditional Chinese with zhuyin and in English, read aloud. See the Taiwan elementary maths curriculum map.