30 mL of 1:400 of solution A containmL = 0.0748 (rounded) mL of the pure substance A. When we add 270 mL of water, the total volume becomes 30 + 270 = 300 mL, with the same amount 0.0748 mL of the pure substance A. Hence, the final concentration is = 0.000249333 = 0.00025 (rounded) = 0.025%.