Two pipes a and b can fill a cistern in 371/2 minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if the b is turned after.

a) 5min
b) 9min
c) 10min
d) 15min

• 'b' should be turned after 21 minutes OR turned on for 9 minutes.
  1 min. capacity of a=1/(75/2)=2/75
  1 min. capacity of b=1/45
  In order to fill the cistern in half an hour or 30 min.,
  'a'will be on for 30 min.,so cistern filled=30*2/75=4/5
  remaining 1/5 is to be filled by 'b',so time required=(1/5)/(1/45)=9 min.
  So 'b' should be on for just 9 min. or turned on after 21 minutes,after 'a'
  
  
• 11 years agoHelpfull: Yes(8) No(1)
• ans 9min
  part filled by (a+b) in x min+ part filled by a in (30-x)min=1
  11x+180-6x=225 solve this equ we get 9min
• 11 years agoHelpfull: Yes(1) No(0)