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

For the first filter, 40 minutes will be takeb to fill 20 litres 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) x 70=60 minutes=1 hour.