A water filter takes 40 min. to filter 20 litres of water. Another filter of same specifications takes 30 min. to filter the same amount of water. If both the filters are used at the same time, then how long will it take them to filter 70 litres of water?

Options
1)1/2 hour
2)3/4 hour
3) 1 hour
4) 1 1/2 hours

• Ans is 3)1 hour
  For the first filter,
  40 minutes take to fill 20 liters of water.
  Therefore,in 1 minute 20/40=1/2 liters of water can be filled.
  
  For the second filter,
  30 minutes take to fill 20 liters of water.
  Therefore,in 1 minute 20/30=2/3 liters of water can be filled.
  So,Two types together can fill in 1 minutes=(1/2+2/3)=7/6 liters.
  Now,if 7/6 liters can be filled in 1 minute then,
  70 liters can be filled in=(6/7)*70=60 minutes=1 hour.
• 7 years agoHelpfull: Yes(20) No(0)
• it would take 40*30/(40+30)=40*30/70 to fill 20 liter of water. then to fill 70 liter of water it would take 40*30/70 *70/20=60 mins or 1 hour.
• 7 years agoHelpfull: Yes(1) No(0)
• 60/5*70/20=42 minutes
• 8 years agoHelpfull: Yes(0) No(9)
• 1hour exactly
• 7 years agoHelpfull: Yes(0) No(0)