After a big storm, Mr.Bloop noticed that a lot of the sand from his volleyball pit had been blown away. He drove his pickup truck down to the local gardening supply yard and purchased 4 truckloads of sand to add to the pit. His truck could carry 500 pounds of sand each time. The sand cost $117 per ton. Originally, it took 12 pickup truck loads to fill the pit. What fraction of the sand had been blown away by the storm?