In the diagram bottom of the image uploaded , it's clear that width of the section after and before the cut are the same , right , which is 25x3=75mm , why the author stated that the width of section after cut become 25mm ? I cant understand
As @JMac pointed out in a comment, the cut in the beam causes the following cross-section to occur (exaggerating the size of the cut):
Originally, the shear stress can flow from the top to the bottom along the three webs. Therefore, the effective width to calculate the shear stress was equal to the sum of the three webs. After the cut, however, the shear can only flow through the central web. This therefore triples the maximum shear stress.