x + 16 = daughter's age in 16 years
4x + 16 = man's age in 16 years
2(x + 16) = 4x + 16 {in 16 years he will be twice as old as his daughter}
2x + 32 = 4x + 16 {used distributive property}
2x = 16 {subtracted 2x and 16 from each side}
x = 8 {divided each side by 2}