A circular hole of diameter d =10 cm is to be punched out of a plate having an ultimate shearing stress, ${\sigma }_s=2\times {10}^{8}pa$  as shown in figure. If the maximum compressive stress on the punch is limited to  ${\sigma }_c=4\times {10}^{8}pa$ .The maximum thickness ‘t’ (in cm) of the plate in which punch can be made