First figure out how much water beans take by multiplying the water use for corn by 2: 20 gallons/acre * 2 = <<20*2=40>>40 gallons/acre
Next, calculate Bob's water use by multiplying each plant's water use per acre by the number of acres he planted, then adding up the total water use for all three crops: 3 * 20 + 9 * 80 + 12 * 40 = <<3*20+9*80+12*40=1260>>1260
Now do the same thing for Brenda: 6 * 20 + 7 * 80 + 14 * 40 = <<6*20+7*80+14*40=1240>>1240
And do the same thing for Bernie: 2 * 20 + 12 * 80 = 1000
Now add up each farmer's water use to find the total water use: 1260 + 1240 + 1000 = <<1260+1240+1000=3500>>3500
Now divide Bob's water use by the total water use and multiply by 100 to find what percentage of the water he used: 1260 / 3500 * 100 = <<1260/3500*100=36>>36%
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