First find the total volume of the pool: 10 feet * 10 feet * 6 feet = <<10*10*6=600>>600 cubic feet. This is also the number of gallons he needs to fill the pool.
Subtract the pool water and the drinking and cooking water from the total amount of water Jerry is allowed to use: 1000 gallons - 600 gallons - 100 gallons = <<1000-600-100=300>>300 gallons
Then divide the amount of water Jerry has left to use for showers by the amount of water per shower to find how many showers he can take: 300 gallons / 20 gallons/shower = <<300/20=15>>15 showers
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