Question 1175217
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Pay attention to the response from tutor @ikleyn, in which she shows an informal solution in addition to a formal algebraic solution.<br>
The process for solving the problem using formal algebra is well defined.  Solving the problem by that method will not require a lot of thinking -- you just follow the well-defined algebraic steps.<br>
On the other hand, solving the problem informally, by using logical reasoning and mental arithmetic, will give you MUCH BETTER exercise for your brain.  That mental exercise will help prepare you for solving problems in the future -- not only in mathematics, but in life in general.<br>
So when you see a problem like this, before you plunge into a solution using formal algebra, give yourself the benefit of taking a few moments to try reasoning out a solution informally.<br>