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article {
    height: 50px;
    width: 48%;
    background-color: silver;
    float: left;
    margin: 1%;
}

article:last-of-type {
    width: 98%;
}

article:nth-of-type(2n + 1) {
    width: 48%;
}

article:first-of-type {
    width: 98%;
}

<section>
    <article>
    </article>
    <article>
    </article>
    <article>
    </article>
    <article>
    </article>
    <article>
    </article>
    <article>
    </article>
</section>