Give the step-by-step reasoning process and then the final answer. Emily is 20 years old and her older sister, Rachel, is 24 years old. How old is Rachel when Emily is half her age?
The difference in age between the two sisters is 24 - 20 = 4 years. When Rachel is twice Emily's age, she is 4 x 2 = 8 years old.
The final answer: 8.