12/27

若 12/27 = x/18 = 28/y,
求 x 與 y.

解一:
留意
12 = 4 * 3
27 = 9 * 3
故 12/27 = 4/9.

因此, 題目等於問
4/9 = x/18 = 28/y.
鑑於 18 = 9 * 2 及 28 = 4 * 7,
可知 4/9 = 8/18 及 4/9 = 28/63.
故 x = 8 及 y = 63.

解二:
左等號兩邊之分母
27 = 3 * 3 * 3
18 = 2 * 3 * 3
鑑於 18 = 27/3 *2.
可見 x = 12/3 *2 = 4 *2 = 8.

其次, 等式首尾之分子
12 = 2 * 2 * 3
28 = 2 * 2 * 7
留意 28 = 12/3 *7.
是故 y = 27/3 *7 = 9 *7 = 63.