用英语来解释下列句子
1.Mark spent fifty seconds finding the answer.
2.Multiply three by seven,and you get twenty-one.
3.Abacuses are fast and accurate,so people still use them today.
參考答案:1.It took Mark fifty seconds to find the answer.
2.seven multiply three is twenty-one.
3.Because abacuses are fast and accurate, people still use them today.