A tank is connected with 7 pipes in total, some of which are inlet pipes and the others are outlet pipes. Each inlet pipe can fill the empty tank in 4 hours while each outlet pipe can empty the full tank in 5 hours. If all the pipes are opened together, it takes 2.5 hours for the empty tank to be filled. Find the number of inlet pipes. 

1. 3
2. 4
3. 5
4. 2 

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https://tathagat.mba/quant-question-of-the-day-131/