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.