Pipe A alone takes 40 minutes to fill a cistern. Pipe A was turned off after working for 15 minutes. The rest of the cistern is filled by pipe B in 45 minutes. How much time in minutes does pipe B take to fill the cistern alone in hours?
A) 72 B) 2 C) 5⅕ D) 4½

• A can fill V volume in 40 min
  i.e...per 1 min V/40 volume
  it worked for 15 mins...-->15*v/40=3V/8 volume is filled by A
  remaining volume=v-3v/8=5v/8
  as per the problem B fills 5v/8 volume in 45 mins
  v volume-->? min
  i.e...45*v/5v/8=72min
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