Are you required to make a solution containing both potassium carbonate and sodium bicarbonate at the stated concentrations, or separate solutions of each which you then mix together? I assume the former, in which case don't make separate solutions at these concentrations and mix them, or you will halve the concentrations.
30.18% w/v means 30.18 g solute in 100 ml of solution. You cannot assume that this contains 69.82 g water, because the density of the solution will be different from 1.
Add 30.18 g potassium carbonate and 6.04 g sodium bicarbonate to a 100 ml volumetric flask. Add enough water to dissolve the salts, but not quite enough to take the volume up to 100 ml (to allow for possible volume change on mixing). If the solution has warmed up or cooled down, allow it to reach room temperature again. Add more water to fill up to the 100 ml mark, and mix thoroughly.