A cistern has three pipes, a, b, and c. Pipe a and b are inlet pipes whereas c is an outlet pipe. Pipes a and b can fill the cistern in 3 hours and 4 hours respectively. If all pipes are opened simultaneously, how long will it take to fill the cistern?
a) 1.5 hours
b) 2 hours
c) 2.5 hours
d) 3 hours
