entities</a>.</p>
                </div>
            </div>
            <div id="section2">
                <div class="unit">
                    <h3>Counting</h3>
                    <p>It can be surprisingly difficult to count the number of sequences or sets satisfying certain conditions. For example, consider a bag of marbles in which each marble is a different color. If we draw marbles one at a time from the bag without replacement, how many different ordered sequences (permutations) of the marbles are