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?
“19/ Anna’s age（年令算）” の続きを読む
13/ ( a+b )( c+d )の説明
( a+b )( c+d ) = ac+ad+bc+bd と計算するのが”普通”だと思いますし、実際自分もこの順で展開しながら計算しています。
これにはそれなりの理由があります。（後述）
12/ Combination（順列）
SATの問題から『順列組み合わせ』を取り上げます。
Q: George is going on vacation and wishes to take along 2 books to read. If he has 5 different books to choose from, how many different combinations of 2 books can he bring?
