Hi there! :)
Answer:
1,800 mg of codeine is needed to prepare 120 mL of a cough syrup with a codeine concentration of 15 mg/mL.
Explanation:
The concentration of codeine in the cough syrup is 15mg/ml, which means that for 1 mL of the cough syrup there is 15mg of codeine.
You want to know how much codeine there would be in 120mL of cough syrup.
In order to answer your question, you'll need to use the cross product method:
1 mL → 15 mg
120 mL → X mg
(15 × 120) ÷ 1 = X mg
1,800 ÷ 1 = X mg
1,800 = X mg
There you go! I really hope this helped, if there's anything just let me know! :)