First, the total thickness of the steel=(4+6+5+3)/16=18/16" when all the individual thicknesses are based on the numbers of sixteenths. The bolt head is separated from the first piece of steel by the washer, which adds another ⅜", that is, 6/16". The nut secures the bolt to the bottom piece of steel and adds another 12/16", and the bolt must extend 4/16" beyond the nut. So we are adding (6+12+4)/16=22/16" to 18/16"=40/16"=5/2"=2½".