Yep!
24mg means 24mg of nicotine per 1 ml of
e-liquid.
Simple example: 10 ml of 24mg (i.e., 240mg) + 10 ml of 0mg = 240mg of nicotine in 20 ml of
e-liquid = 240mg / 20 = 12mg per ml.
More complicated example: 30 ml of 36mg + 15 ml of 12mg = (1080mg + 180mg) / (45 ml of
e-liquid) = 1260 / 45 = 28mg per ml.