Given the task definition and input, reply with output. In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Problem: an empty tank be filled with an inlet pipe ' a ' in 42 minutes . after 12 minutes an outlet pipe ' b ' is opened which can empty the tank in 30 minutes . after 6 minutes another inlet pipe ' c ' opened into the same tank , which can fill the tank in 35 minutes and the tank is filled . find the time taken to fill the tank ?
Options: a ) 51.6 , b ) 51.5 , c ) 51.7 , d ) 51.1 , e ) 51.0
b