A processor of carrots cuts green top off each carrot, washes the carrots, and inserts six to package. Twenty packages are inserted in a box for shipment. To test weight of the boxes, a few were checked. The mean weight was 20.4 pounds; the standard deviation, 0.5 pounds. How many boxes should the processor sample to be 95 percent confident that sample mean doesn't differ from population mean by more than 0.2 pounds?