To find out how many pints are in 20 gallons of water, we first need to know how many pints are in one gallon.
1 gallon is equal to 8 pints.
So, to find the total number of pints in 20 gallons, we multiply:
\[ 20 \text{ gallons} \times 8 \text{ pints/gallon} = 160 \text{ pints} \]
Maddie collected 160 pints of river water.