The sum of the ages of a mother a father and their son is 100 years. Each of their ages is a multiple of 10 and the mother was 30 years old when her son was born. How old is the son?小兒子的數學題, 英語不算難. 爹媽和兒子的年齡之和是100, 三人的年齡都是10的倍數. 媽是在30歲時生的兒子.問兒子幾歲?解: 設 [ ...閱讀全文 ]