To find the percent of the yellow tint in the new mixture?

1)We need to know the amount of yellow tint in the old mixture.

amount of yellow tint=30% of 40 liters
30%=30/100=0.3

Amount of yellow tint=0.3(40 liters)=12 liters.

2)We need to find the amount of yellow tint in the new mixture.

amount of yellow tint in the new mixture=12 liters + 9 liters=21 liters.

3) we calculate the amount of the new mixture:
amount of new mixture=40 liters + 9 liters=49 liters

4) we compute the percent of yellow tint in the new mixture:

percent of yellow tint =(amount of yellow tint / amount of mixture)*100

percent of yellow tint=(21 liters / 49 liters)100≈42.9%

The percent of yellow tint in the new mixture is 42.9%. 

note: if this answer is not correct you can try it  with 42.8%