math · 26 minutes · Official OETT Aligned
Flow Rate
How quickly a tank fills or drains
What to know
- rate: amount per minute or second
- volume: amount of liquid
- time to fill
- flowing in and flowing out
The units help you decide whether to multiply or divide.
The core rule
Rate = volume ÷ time. Volume = rate × time. Time = volume ÷ rate.
Gallons per minute tells you how many gallons move each minute. If water enters and leaves a tank at the same time, subtract the drain rate from the fill rate. Then use the amount still being added each minute to find the filling time.
Worked examples
Example 1: easy
360 gallons in 12 minutes is 30 gal/min.
- Write rate = volume ÷ time.
- Calculate 360 gal ÷ 12 min = 30 gal/min.
Gallons per minute means gallons divided by minutes.
Example 2: medium
A 480-gallon tank at 40 gal/min fills in 12 minutes.
- Write time = volume ÷ rate.
- Calculate 480 gal ÷ 40 gal/min = 12 minutes after gallons cancel.
Divide the amount to fill by the amount added each minute.
Example 3: hard
A 50 gal/min inlet and 15 gal/min drain net 35 gal/min.
- Treat inlet as +50 gal/min and drain as −15 gal/min.
- Net rate is 50 − 15 = 35 gal/min.
Subtract what drains out from what flows in.
Common mistakes
The word per always means multiply.
Per means for each. To find gallons per minute, divide gallons by minutes.
Put it into practice
Use guided explanations, check your answers, and try a new variation.