At the beginning of the rainy season, the tank is filled with 100 x 2/5 = <<100*2/5=40>>40 liters of water.
On the first day of the rainy season, the tank had 40 + 15 = <<40+15=55>>55 liters of water.
On the second day, 15 + 5 = <<15+5=20>>20 liters of water were collected.
So, the tank is filled with 55 + 20 = <<55+20=75>>75 liters of water on the second day.
Therefore, 100 - 75 = <<100-75=25>>25 liters of water were collected on the third day.
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