8 liters of saline water with salt content of 10% contains 8*( 1- 10%)=7.2 liters.

The physiological saline containing 20% salt is 7.2( 1-20%)= 9 liters.

Need to add salt 9-8= 1 liter.

Option 2: Water evaporation

Salt content 8* 10%=0.8 liter.

Salt water with a salt content of 20% is 0.8÷20%=4 liters.

Need to evaporate water 8-4=4 liters.