An iv solution containing 1000 mg of aminophylline in 1000 ml of solution will completely infuse in 24 hours. You can write the rate as either 1000mg/24hour or 1000ml/24 hour.
You are asked how many mg aminophylline infused per hour. To find it you just need to divide the rate and makes the denominator become 1 hour.
The calculation would be: 1000mg/24hour = 41.66 mg/1 hour
