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

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TacoGodid
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. 

Crisforpid
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);