Wastewater · Worked step by step
What is the detention time of a 100,000 gallon tank at 25,000 GPH?
If math has never been your thing, you are in the right place. Try it on paper first, then we write it out with you, one small step at a time, and explain every step in plain words so you never lose the thread.
The problem
A tank holds 100,000 gallons and the flow through it is 25,000 gallons per hour. What is the detention time in hours?
Grab a pen and paper first
Give it a real try on paper before you watch. If you get stuck, take a hint. If you go blank, that is fine, press play and we will walk it through together.
Watch it solved, with you
Every step is written out slowly and explained in plain words, so you always know what just happened. Nobody skips ahead and leaves you behind.
Press play and I’ll write it out with you, one small step at a time.
Every step gets a plain-words explanation here, so you always know exactly what just happened and why.
The written version, step by step
- 1
Write what you know
volume = 100,000 gal • flow = 25,000 gal/hour
- The tank holds 100,000 gallons.
- 25,000 gallons flow through each hour.
- Per hour, so the answer is in hours.
- 2
Divide volume by flow
time = 100,000 ÷ 25,000 • time = 4 hours
- Detention time = volume divided by flow.
- How many 25,000s fit in 100,000? 25,000 × 4 = 100,000.
- So the detention time is 4 hours.
Quick trick: Cancel three zeros from each: 100 ÷ 25 = 4.
Remember this for good
Detention time = volume divided by flow. Gallons over gallons-per-hour gives hours.
Volume on top, flow on the bottom. Ask how many times the hourly flow fits into the tank; that count is the hours.
Solve it fast next time
- Detention time = volume divided by flow.
- Gallons divided by GPH gives hours.
- Cancel matching zeros to divide big round numbers fast.
The answer: 4 hours.