S = F/3                  (1)

   S+25 = (F+25) / 2        (2)



(1) —>  3S = F

(2) —>  2S+50 = F+25 —> 2S+25 = F



Subtract the last version of (2) from the last version of (1) (that makes F vanish):



            3S - (2S+25) = 0

                 S - 25 = 0

               S =25  —> F (=3S) = 75

—

Ans:  Sue's father is  years old

—

Check:   in 25 years, Sue will be 50,  and her father will be 100,  and 100/2 = 50 (ok) 

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