Question 1203578
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Being able to solve the problem using formal algebra, as a couple of the other tutors did in their responses, is an important mathematics skill.  And perhaps that is what you were expected to do.<br>
But doing that mostly gives you practice in following well-defined processes; it doesn't do much to improve your skill at problem solving.<br>
For good problem-solving practice and excellent mental exercise, solve the problem mentally, as tutor @ikleyn did.<br>