There are two inlet pipes 'A', 'B', and one outlet pipe 'C' attached to a tank. pipe 'A' and 'B' can fill a tank in 6t hours and 8 hours respectively and pipe 'C' can empty the tank in 4t hours. If all three pipes are opened together and 700 liters of the water is already in the tank then after (2t + 1) hours 75% of the tank will be filled. What is the value of '2t' if the total capacity of 3500 liters?