Mr. Green is a famous writer now. But he said he was not a good student when he was young. He was often late for school and didn't like doing his homework. Sometimes he slept in class while the teacher was teaching. He didn't understand much, but he always thought he understood everything. One day the teacher asked the students a question, "When John was ten years old, his brother was twenty. John is fifteen now and how old is his brother?" Mr. Green answered, "That's easy. His brother is twice as old as John, so he is now thirty." Another time, the teacher in a science class asked, "When it thunders , why do we always see the light before we hear the sound?" "But, Miss White," said Mr. Green quickly, "can't you see our eyes are in front of our ears?" The third time, the teacher in a biology class asked, "Why can fish swim in the water?" "But, Miss White," said Mr. Green quickly, "don't you know fish can't walk on land?" How old is John's brother when John is fifteen in the teacher's question?
10 years old. 15 years old. 20 years old. 25 years old.