For the first filter
40 minutes will be taken to fill 20 litres of water.
Therefore, in 1 minute
20/40=1/2 litres of water can be filled.
For the second filter,
30 minutes take to fill 20 litres of water.
Therefore, in 1 minute
20/30=2/3 litres of water can be filled.
So, Two types together can fill in 1 minute
=(1/2+2/3)=7/6 litres.
Now,
if 7/6 litres can be filled in 1 minute then, 70 litres can be filled in
=(6/7) x 70=60 minutes
=1 hour.
Answer
It will take 1 hour to filter the 70 litres of water