Number of riser in each flight

Given, height of each flight is half of vertical distance between the floors, hence height of each flight is 3.6/2   or 1.8 m

& Height of single riser = 15 cm or 0.15 m

∴ Number of riser in each flight = 1.8/0.15= 12  

∴ Number of treads in each flight 

                                = Number of riser – 1

                                = 12 – 1 = 11