as vy3rgc mentioned, 1/2:4 ratio is a 1:8 ratio.
I figured it out this way.
in a 10 part solution, there is 2 bleach and 8 water.
Jim only added 1/2 the amount of bleach needed so instead of 2 bleach he added 1 bleach and 8 water.
This also changes it from a 10 part mixed solution to a 9 part mixed solution.
In a 18 part solution with this mistake, he'll have 2 parts bleach and 16 parts water.
I figured it out this way.
in a 10 part solution, there is 2 bleach and 8 water.
Jim only added 1/2 the amount of bleach needed so instead of 2 bleach he added 1 bleach and 8 water.
This also changes it from a 10 part mixed solution to a 9 part mixed solution.
In a 18 part solution with this mistake, he'll have 2 parts bleach and 16 parts water.