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Math Help - percentages question

  1. #1
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    percentages question

    I run an irrigaiton system. I have a limited amount of water available for the year (100,000,000 gallons). We have broken it down so that we know we want to apply a certain percentage of the total water available per month: In Dec, Jan, and Feb the system will be off so no water is alloted. Mar=7.3% of the total available water, Apr=10.3%, May=13.1%, Jun=13.7%, Jul=19.4%, Aug=19%, Sep=11.1%, Oct=3.5% and Nov=2.6%

    I can adjust the run times of water zones on a percentage basis, therefore adjusting the amount of water applied. I know that when I run my system at 85% for a month it equates to 19.4 million gallons of water applied. So for the month of August I can set my system to 85% of normal and I will use the alloted 19 million gallons of water.

    What formula should I use to determine what to set the system % to in other months to not exceed my alloted water levels?

    Does taking the % for a particular month and dividing it by 19% then multiplying it by 85% work?

    have I given enough information to solve this?

    thanks,

    Justin
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  2. #2
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    Are the relationships linear ?

    If the system were to be run at 85% in, say, March and October, (too much water I know), would the amount of water supplied be 19.4 million gallons on both occasions, or does the amount supplied depend upon the amount available ?

    If the sytem were run at, say, 17% would the amount of water supplied be 19.4/5 = 3.88 million gallons ? (85/5 = 17).
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  3. #3
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    I guess they are linear

    The water is always available, it is the money to pay for the water that is in short supply. So yes, no matter when the system is set to 85% we will use 19.4 million gallons of water for a month.

    If the sytem were run at, say, 17% would the amount of water supplied be 19.4/5 = 3.88 million gallons ? (85/5 = 17).

    I guess that is my question.

    It seems like if I know that setting the system to 85% causes it to use 19.4 % of my total available water then the following formula works to figure out what to set the system at to use 3.5% of the available water:

    .035/.19*.85 = new percentage

    Yes? It's been a long time since I've had any math classes.

    thanks for your help
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