Answer:
It was first done by William Jones in 1706
Girl age is 11 years old
Fathers age is 39 years old
How to solve it:
Let the girls age be x years
Let the fathers age be (x+28) years
X+X+28=50
2x=50-28
2x=22
X=11
(11+28)= 39
Instead of subtracting 10, you can add negative 10