(1) Because it is prepared according to the ratio of 3:400, it can be seen that there are 3 parts of powder and 400 parts of liquid medicine, so the total amount of liquid medicine to be prepared is 3+400=403 parts, and the liquid medicine of 16 12g is to be prepared, so 1665438 is used.

(2) Because you need 18g powder, and because it is 3:400, you need 3 parts of powder, so 18 is divided by 3 = 6g, so each part is 6g, and you need 400 parts of water, so 6 times 400 = 2400g of water, and powder1.