An industrial solvent of 90% strength is stored in a container of capacity 150 liters. The container is filled to 80% of it's capacity.Now it's required to decrease the concentration to less than 60%. for doing this either pure water can be added or solution can be removed. But removal can only be done when there is no place for adding water.at a given time only 10 liters of solution can be added or removed.How many times water should be added to solution?

1) 4
2) 5
3) 6
4) 7
5) 9
6) Better leave it.