Question

Katherine is three times older than Joel now. In twenty years Katherine will be six years older than Joel. How old is Katherine now?

Answer 1

k = 3*j;
k + 20 = 6 + (j + 20) => k = 6 + j; but k =3*j => 3*j = 6 + j => j = 3( Joel has 3 years) and k = 9 (Katherine has 9 years);

Answer 2

She is currently 9 years old.

If Katherine is 9, and Joel is 3, 20 years later they'll be 29 and 23. Katherine is 6 years older than him.