19/ Anna’s age（年令算）

Q: 　Maria is 24 years old. It is twice as many years as Anna had when Maria was as old as Anna is now. How old is Anna?

[解説]     Let Y be the age of Anna in years.（Annaが Y歳であるとしましょう）・・・・こうやって文字でおけば計算できますからね。

24 = 2 x 12　なので　Anna was 12 years old when Maria was as old as Anna is now.　　言い換えれば

Anna が12歳の時、Maria はY歳であり、それが今は

Anna がY歳で、Maria は24歳です。　ポイントは

The difference in their ages remains constant. （歳の差は一定）

よって、　24 – Y = Y – 12   であり、これを解けば　Y = 18

Anna is 18 years old.