SOLUTION: bon wants to take an early retirement in 10 years when he will be twice as old as his daughter Sonia. Sonia is 25 years younger than her father. How old is bon now? hoe old will he

Let x be the Bon's age and let y be the Sonia's (his daughter's) age.


From one side, we have this equation 

x - y = 25            (1)    ("Sonia is 25 years younger than her father".)



From the other side, we have second equation

x + 10 = 2*(y+10)     (2)    (in 10 years . . . )


So, we have the system of equations (1) and (2) in two unknowns.

To solve it, express  x = 25 + y  from eq(1), and substitute it into equation (2). You will get

(25 + y) + 10 = 2*(y+10)

35 + y = 2y + 20

35 - 20 = 2y - y

15 = y.


Thus Sonia is 15 years old now.

Hence, the father is  x = 25 + y = 25 + 15 = 40 years old.


Answer.  Sonia is 15 years old;  the father is 40 years old.

Let x be the Sonia's age.

Then the Bon's age is  (x+25), according to the condition.


In 10 years Sonia's age will be (x+10) years,  while Bon's age will be (x+25)+10 = (x+35).


Then the condition says

x + 35 = 2*(x+10).


It implies

x + 35 = 2x + 20

35 - 20 = 2x - x

15 = x.


And you get THE SAME ANSWER.