NG output question  Med Calc!!

I am working on a med calc study guide to prepare me for a med calc test coming up. I need help on a question, which I have the answer for, I just do not know how to get that answer. Thanks so much for any help or guidance!
Ordered:
NG tube to low continuous suction.
Maintenance IV: D5W/0.45% NS with 20 mEq KCL per 1000 mL to run at 100 mL/hr
Measure NG output and replace with 2nd IV of D5W/0.45% NS with 10 mEq KCL per 1000 mL
Total hourly IV fluid intake not to exceed 125 mL/hr
Question: The patient had an NG output of 100mL from the previous shift. How many hours will it take to replace the NG output at an IV rate not to exceed 125 mL/hr?
Answer: 4 hours (Now, I just don't know how to arrive at that answer!!)
Thanks again!! 

Jan 27, '13Quote from bhofer009Ok, your maintenance fluid is running at 100 mL/hr. Your max rate is 125 mL/hr. So the fastest you can run your replacement fluid is at 25 mL/hr, right? So you have 100 mL you need to replace. 100 mL divided by 25 mL/hr = 4 hrs.I am working on a med calc study guide to prepare me for a med calc test coming up. I need help on a question, which I have the answer for, I just do not know how to get that answer. Thanks so much for any help or guidance!
Ordered:
NG tube to low continuous suction.
Maintenance IV: D5W/0.45% NS with 20 mEq KCL per 1000 mL to run at 100 mL/hr
Measure NG output and replace with 2nd IV of D5W/0.45% NS with 10 mEq KCL per 1000 mL
Total hourly IV fluid intake not to exceed 125 mL/hr
Question: The patient had an NG output of 100mL from the previous shift. How many hours will it take to replace the NG output at an IV rate not to exceed 125 mL/hr?
Answer: 4 hours (Now, I just don't know how to arrive at that answer!!)
Thanks again!! 
Jan 27, '13Thank you! That's what I thought, but then I thought it was too simple. Guess I should be used to that by now. That is very helpful!!