Question

Kim's age is twice that of her sister. When you add Kim's age to her sister's age, you get 36. How old is each sister?

Answer 1

The answer is: Kim - 24 years old. Sister - 12 years old.

K - Kim's age
S - sister's age

Kim's age is twice that of her sister: k = 2s 
When you add Kim's age to her sister's age, you get 36: k + s = 36

k = 2s
k + s = 36
_______
2s + s = 36
3s = 36
s = 36 : 3
s = 12

k = 2s
k = 2 * 12
k = 24

Answer 2

k+s = 36
k = 2s
substitute for k above:
 2s+s = 36
3s = 36
s = 12 and k = 2s
so
 k = 2(12)= 24

So kim is 24 and sister is 12.