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<h3>a) Add columns by reference</h3>
<div id="how-can-we-add-columns-speed-and-total-delay-of-each-flight-to-flights-data.table" class="section level4">
<h4>– How can we add columns <em>speed</em> and <em>total delay</em> of each flight to <code>flights</code> <em>data.table</em>?</h4>
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true"></a>flights[, <span class="st">`</span><span class="dt">:=</span><s