Normal saline 100g, with a concentration of 20%;

The amount of salt is 100×20%=20 (g).

The water quantity is100-20 = 80g.

The amount of water remains unchanged, and the concentration of salt water becomes 25%.

The amount of salt is 80× 1/3=80/3 (g).

The addition of salt is 80/3-20=20/3 (g).

Method 2: Distillation to reduce water.

Normal saline 100g, with a concentration of 20%;

The amount of salt is 100×20%=20 (g).

The water quantity is100-20 = 80g.

The salt content is unchanged, and the salt water concentration becomes 25%.

The water quantity is 20× 3 = 60g.

The amount of water to be evaporated is 80-60=20 (g).