The denominator only changes if you're multiplying/dividing. You may need to create an <em>equivalent fraction</em> to add the fractions together, but you <em>don't</em> add or subtract the denominator.
Since the first type of measurement in this question is weight or mass, I'll suppose that the percentage concentration is % mass/mass. For that type of concentration measurement, just multiply the percentage by the total mass to get the mass of the wanted material. So 150 g * 3% = 150 g * 0.03 = 4.5g
For the 8% solution with the same amount of dry substance, use the ratio of percentages, multiplied by the mass of the first solution to get the wanted amount of new solution: 3/8 * 150 g = 56.35 g
