Lets Anna's age = A
Mr. Goldstein age = G
Mr. Goldstein is 4 times as old as his daughter Anna.
So G= 4A
In 4 years, he will be 3 times as old as Anna
we add 4 with Both their ages
G + 4 = three times of A + 4
G + 4= 3(A + 4)
We know G= 4A, replace it in above equation
4A + 4= 3(A + 4)
4A + 4= 3A + 12
Subtract 3A from both sides
A + 4 = 12
Subtract 4 from both sides
A = 8
So Anna's age = 8 years
