Well if I did the right calculations... Kim is 24. The sister is age twelve! Hope I helped.
A. K+S=36
B. Well Simple Addition and Subtraction solves this easily. If you Add 12+12 it equals 24. It still needs to be larger so that Kim is twice as old as her little sister. So you Add another 12 which gives you 36. Or you can easily subtract 12 from it and that will also bring you to 24.