Dimentional Analysis question: Order for Tylenol 3/4 tsp (120 mg/5ml) is ordered q6h prn for?

Question

pain. On hand is 200mg/5ml. how many ml's will you administer?

I know the answer is 2.25 mL... HOw do you come up with that answer?

ps. im not a pharmacist or anything, its a class im taking, so dont freak out thinking your pharmacy comes on here asking beginner questions! lol

Answer 1

Order is for 3/4 tsp. of Tylenol [ 120mg./5ml ].
Divide 3/4 into 5 ml, and 3/4 into 120 mg.
3/4 tsp. = 3 and 3/4 ml. = 90 mg.
A ratio formula of 90 mg./x = 200 mg./5ml
Crossmultiply.
450 = 200 x
Divide.
2.25 = x