To be consistent, I will use the term dosage to express the amount of medication which is given in mg. The medicine containing that dosage is the medication and it is given in ml.
Dosage in mg.
Medication in ml.
You can use a proportion to solve this problem. A proportion is setting two ratios equal. First, use the dosage in mg and the medication amount in ml that are both known to set up a ratio.
Known: 1.5 ml of medication has 25 mg of dosage.
The ratio of medication to dosage is 1.5 ml to 25 mg, or as a fraction, the ratio is: (1.5 ml)/(25 mg)
Now we work with the ratio that has an unknown. Let's call the unknown volume of medication x. x ml of medication contains 20 mg of dosage. The ratio here must contain x since we don't know what the volume of the medication is. The ratio is x ml to 20 mg or (x ml)/(20 mg) as a fraction.
To set up a proportion, we set the ratios equal.
First, let's drop all units since we know we are solving for a volume in ml.
Now we cross multiply to solve for x.
Answer: 1.2 ml of medication must be administered.