###Human: Mandy is 3 years old. Her brother is 4 times as old as she is. Her sister is 5 years younger than her brother.  What is the age difference between Mandy and her sister?, ### Assistant: Mandy's brother's age is 3 years x 4 = 12 years old.
Her sister's age is 12 years - 5 years = 7 years old.
The age difference between Mandy and her sister is 7 years - 3 years = 4 years. 
 So the answer is :  4