Count nr of divs apply class to every fifth

Count nr of divs apply class to every fifth

Hi, i have a structure which looks like

  1. <row>
    <div class="onecol"></div>
    <div class="onecol"></div>
    <div class="onecol"></div>
    <div class="onecol"></div>
    <div class="onecol last"></div>

    <div class="onecol"></div>
    <div class="onecol"></div>
    <div class="onecol"></div>
    <div class="onecol"></div>
    <div class="onecol last"></div>
    </row>
all divs are generated dynamic, so the amount can vary from 5 to 25 divs. Is it possible to set a rule saying, apply class "last" to every fifth div with the class "onecol"? I need it to count from left to right.