Stumped on this math question

Total per day would be the 20mg x18 kg....360/ day, which you calculated....divide by 3 for the dose. Same with the 40 mg. you'll then have a range for both total/day and dose. You did it right.

ok thank you so much

Low dose range: 18kg x 20mg/day

High dose range: 18kg x 40mg/day

the answers are your safe dose ranges per day: 360mg/day - 720mg/day

To get per dose divide 360mg and 720mg by 3 and that will be: 120mg - 240mg q8h

You thought about it correctly. There's a safe lower limit 24 hour dose at 20mg/kg, and a safe upper limit dose for 40mg/kg. Those are your daily totals. Divide each by three for lower and upper individual doses.

Do not get too married to "how to set it up." We see people all the time who try valiantly to stuff every number in a problem into a dimensional analysis formula, and often enough, it's not necessary to do that. The people who write exam questions know this, and give three wrong answer choices that will result from putting the wrong numbers in the wrong places, knowing that some people will choose them. You, on the other hand, have correctly thought about what the problem is actually asking you, and discerned the best way to figure out the answer. Never lose this ability.