Question 1199657
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It's misleading for your teacher to ask about THE equation, because many equations can be formed. 
Here's one way to go about it.


x = Julie's age
x+3 = Janet's age since "<font color=blue>janet is three years older than her sister julie</font>"
x-8 = the brother's age since "<font color=blue>janet's brother is eight years younger than their sister julie</font>"


Notice how Janet and her brother have their ages dependent on Julie's age.


These three ages sum, or add, to 55 years in total.


Julie + Janet + Brother = 55
(x) + (x+3) + (x-8) = 55
<font color=red>x + (x+3) + (x-8) = 55</font> is one possible equation to set up.


I'll let the student finish up solving the equation.
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