An osmole (Osmol) is 1 mol of particles that contribute to the osmotic pressure of a solution.
For example, dextrose is a nonelectrolyte.
1 mol of dextrose gives 1 mol of particles in solution. So.
"1 mol dextrose = 1 Osmol dextrose"
Thus, the osmolarity of the dextrose in your solution is
"Osmolarity" = (20 color(red)(cancel(color(black)("mmol"))))/(100 color(red)(cancel(color(black)("mL")))) × (1000 color(red)(cancel(color(black)("mL"))))/"1 L" × "1 mOsmol"/(1 color(red)(cancel(color(black)("mmol")))) = "200 mOsmol/L"
"KCl" dissociates completely in water to form "K"^+ ions and "Cl"^"-" ions.
Thus, each mole of "KCl" becomes two osmoles in solution: one of "K"^+ and one of "Cl"^"-".
The osmolarity of "KCl" in your solution is
"Osmolarity" = (4 color(red)(cancel(color(black)("mmol"))))/(100 color(red)(cancel(color(black)("mL")))) × (1000 color(red)(cancel(color(black)("mL"))))/"1 L" × "2 mOsmol"/(1 color(red)(cancel(color(black)("mmol")))) = "80 mOsmol/L"
The total osmolarity of the solution is the sum of the individual osmolarities.
"Total osmolarity" = "(200 + 80) mOsmol/L" = "280 mOsmol/L"
