What is Mr. White?

Mr. White, a maths teacher, is very strict with his students. When he has lessons, the students must keep very quiet, otherwise he will punish them.
    Joan and Rose arc good at maths, but both of them have one shortcoming: they can not keep quiet in class. They always talk to each other, play with something and so on. This always makes their teacher angry.
     One day when they talked to each other again in class, Mr. White got so angry that he suddenly put his book on the teacher’s desk heavily and said, "Two women is equal to one thousand ducks!" The students couldn’t help laughing. Joan and Rose’s faces went red. From then on, they didn’t make any noise in class at all. Mr. White was very pleased.
    Several weeks later, Mr. White was teaching one day when Joan stood up and said, "Mr. White, five hundred ducks are coming outside the window!" Mr. White was very surprised. He stopped teaching and looked out of the window. He saw a woman coming.
    It was his wife!