&gt; For example, what if she was given 216 phones to fix per week, and she fixes 46 per day?<p>I&#x27;m sorry; I&#x27;m confused.  In that case, the equation would say that there are P = 216 - 46d phones to fix after d days.  This isn&#x27;t the same equation as the given P = 108 - 23d, although its right-hand side is proportional to the right-hand side of the original.<p>(That is, I&#x27;d agree with you if we were given just the ratio (phones to fix per week)&#x2F;(phones fixed per day), but we&#x27;re given more than that.  Perhaps it&#x27;s important to notice that we are given an <i>equation</i> (left-hand side = right-hand side), not just a <i>formula</i> (right-hand side by itself).)