John has a very inefficient toilet that uses 5 gallons of water per flush and his household flushed 15 times per day.  He replaced it with a toilet that uses 80% less water per flush.  How much water did John save in June?
He used to use 5*15=
<gadget id="calculator">5*15</gadget>
<output>75</output>
75 gallons of water per day.
This means he used 75*30=
<gadget id="calculator">75*30</gadget>
<output>2_250</output>
2250 gallons a month.
His new toilet uses 5*.8=
<gadget id="calculator">5*.8</gadget>
<output>4</output>
4 gallons less water per flush.
So, it only uses 5-4=
<gadget id="calculator">5-4</gadget>
<output>1</output>
1 gallon per flush.
That means he uses 1*15=
<gadget id="calculator">1*15</gadget>
<output>15</output>
15 gallons per day.
All in all, it accounts for a total usage of 15*30=
<gadget id="calculator">15*30</gadget>
<output>450</output>
450 gallons.
So, he saved 2250-450=
<gadget id="calculator">2250-450</gadget>
<output>1_800</output>
1800 gallons per month.

<result>1_800</result>