Maximum tensile and compressive stress in a T-beam

Where does the T-beam come from? There's just a straight beam resting on two supports. And where do 10 and 80 come from? I am so confused.

• 80 is width of top surface... May 13 '19 at 18:36
• The T-profile is the shape of the intersection, if you cut the beam at every arbitrary position and look at the intersection you'll see a T. May 13 '19 at 18:51
• About 80, look at the comment of Solar Mike, and 10 is the half of the thickness of horizontal body, the centroid of the horizontal body lies at the center of of the horizontal rectangle $d_{zz}$ is the distance between the two centroids. May 13 '19 at 19:06