Question 1133195
x = 150 + 25 is equivalent to x - 25 = 150  (just add 25 to both sides to see this) <br>

I think his teacher is expecting to see the "proper" starting formulation of 
x - 25 = 150, which indeed makes a little more sense from the problem description.       

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As an example, consider this simple age problem: <br>
John is 6 years younger than Alan
Alan's age and John's age add to 70
Find Alan's age<br>

Let Alan's age be x
John's age is then: x - 6

As a teacher, I would expect the starting formulation to be:
(x) + (x-6) = 70<br>

One wouldn't expect the student, instead, to create a starting formulation of:<br>  2x = 76 <br>  even though they are equivalent.

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Just to follow up:  I used "proper" loosely to mean "most representative of the problem described."   As tutor Boreal said, it is easy to make a mistake if one starts solving the problem in their head.  <br>  

You know the starting point should be x-25=150 because the problem asks for the original price and it says "$25 off the original"  (off ==> subtraction).  <br>

Basically your son partly solved the equation "x-25=150" in his head to arrive at the equivalent "x=150+25"  which will be error-prone for more complex problems.