Jack leaves his bathtub's faucet dripping at a rate of 40 ml/minute. Water evaporates from the bathtub at a rate of 200 ml/hour. If he leaves the water running for 9 hours, then dumps out 12 liters, how many milliliters of water are left in the bathtub?
First find how much water fills the bathtub per hour: 40 ml/minute * 60 minutes/hour = 
<gadget id="calculator">40*60</gadget>
<output>2_400</output>
2400 ml/hour.
Then subtract the water that evaporates to find the total amount of water added per hour: 2400 ml/hour - 200 ml/hour = 
<gadget id="calculator">2400-200</gadget>
<output>2_200</output>
2200 ml/hour.
Then multiply that amount by the number of hours to find the total amount of water in the tub: 2200 ml/hour * 9 hours = 
<gadget id="calculator">2200*9</gadget>
<output>19_800</output>
19800 ml.
Then find the number of milliliters in 12 liters: 12 liters * 1000 ml/liter = 
<gadget id="calculator">12*1000</gadget>
<output>12_000</output>
12000 ml.
Then subtract the water Jack removes from the total amount of water to find the remaining amount of water: 19800 ml - 12000 ml = 
<gadget id="calculator">19800-12000</gadget>
<output>7_800</output>
7800 ml.

<result>7_800</result>