Happy little doctor, happy heart class, the significance of practicing equations in the fourth class of the fifth grade of mathematics (1) How to do this problem in life!
Let age x population y be the result =5(2X+5)+Y, that is, 10X+25+y. Since the end of10x must be 0, the last digit of 10X+25 is 5, plus population. Since the population generally does not exceed 10, we only look at the end of the result. If it is 7, it is two people.