Ok!  This should be easy.<p>&quot;Linear function&quot;: f(x) = mx+b, or y=mx+b.  You know, from 8th-grade algebra.<p>&quot;Baseline and per-unit cost&quot;.  So your &quot;baseline&quot; cost is how much you have to pay <i>before</i> you get the first unit of your item.  Like, if your buying, say, chocolates, the &quot;baseline&quot; is the cost of the <i>box</i>, with no chocolates in it, or of shipping-and-handling.  Then the per-unit costs is just how much you pay for <i>each</i> chocolate.<p>We then map those onto the math:<p>&quot;Baseline&quot; -- that&#x27;s the y-intercept, or &quot;b&quot;.<p>&quot;Per-unit cost&quot; -- that&#x27;s the multiplier for the number of units, or &quot;m&quot;.<p>And voila, we&#x27;ve got y=mx+b: a simple formula for how much your Christmas chocolates are going to cost you.