B$5$ A drainage system design was proposed having a constant slope of $0\mathrm{.}0025$ with three circular channels made of finished concrete. Two of the channels have a diameter of $1\mathrm{.}8$ m and drain into the third channel. If all channels are to run half$-$full and the losses at the junction are negligible, determine the diameter of the third channel. $[10$ marks$]$

solve this in a piece of paper handwritten and post the picture of solution.