1. 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?

Hmm, this is a tricky one. I'll do my best, though.

Well, we know that __ + __ = 36 and that one of the 2 numbers is double the other. We'll use x to be one number, and we'll use 2x to be double the number (because when you double x, you'll have 2x). And we know that we add them, right?
x + 2x = 36. Now this is making sense! Just solve it!

x+2x=36
3x=36
x=12 But don't forget, that's only the age of the younger sister. The older sister is double the age of x, or 12. What is 12 doubled? 24. 

The age of the two sisters is 12 & 24.

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