3. You receive an order for 30 mL of 3% boric acid solution. The pharmacy has in stock a 1% and 5% boric acid solution in stock

Answer 1

.01 x + .05(30 - x) = .03 (30)

Answer 2

or just use the 5% solution and add water. Since 3% is 3/5 of 5%, just use 3/5 of the 30mL and fill the rest with water:

18 mL 5% + 12 mL water

Or, for a much slower method, pour some 1% solution in a beaker and let it evaporate. Since 1% is 1/3 as much as 3%, start out with 3 times the volume. Let 90mL evaporate till only 30mL is left. Then it will be 3%.

Just sayin' ...