Is there a year when the son is 13 and the father is 40? The father's age is 4 times of the son's? Why is it negative!

Is there a year when the son is 13 and the father is 40? The father's age is 4 times of the son's? Why is it negative!

40-13 = 27 years old
Let's set the year when our son is x years old and the year when our father is (x + 27) years old
x+27=4x
4x-x=27
3x=27
x=9
（x+27）=9+27=36
A: when my son was 9 and my father was 36, my father was four times as old as his son
Calculation method:
40-13 = 27 years old
27 / (4-1) = 9 (years old)
27 + 9 = 36 years old
A: when my son was 9 and my father was 36, my father was four times as old as his son
In this problem, the age difference between father and son is constant,
No matter how many years, the age difference will not change
The arithmetic method is to regard the age of the son as a unit: "1"
Find out the age of the son and the age of the father
As for why you find a negative number, it depends on how your formula is listed. Compare the formula on the computer to see what is wrong
Hope to adopt!