neglect my second reply !
:P
see there is a tax on sales , i.e on total revenue.
so the profit would be = TR - 0.20TR - TC
now, profit would be = 0.80x (20-4x) - 4x -2
diff w.rt x to get optimal x
arey its matching ! :/
.80x(20-4x) - 4x-2
16x- 3.2x^2- 4x -2
diff w.r.t x gives
16 - 6.4x -4
putting it equal to zero
12 = 6.4x
implies x = 12/6.4 = 1.875 and this approx 1.9 !!
@ashima I think x is the quantity over here. 0.2 of x does not make sense to me. Sales tax will be applied to price only and applying to Selling price also makes sense. Will have to look at dead weight loss for these question.