At 2 pm there are 2 inches of rain in the tank.
At 6pm, there are 2 + 4 * 1 = <<2+4*1=6>>6 inches of rain in the tank.
That means that there are 18 - 6 = <<18-6=12>>12 inches of tank that still need to be filled as of 6 pm.
It will take 12 / 3 = <<12/3=4>>4 hours to finish filling the tank.
The tank will be filled at 6 + 4 = <<6+4=10>>10pm.
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