ANSWER
66.67%
Step-by-step explanation
We have that there were 6 volunteers in the experiment and 4 do not receive any treatment.
To find the percent of volunteers that do not receive any treatment, we have to divide the number of people that do not receive treatment by the total number of people that were in the experiment and multiply by 100.
That is:
That is the percent of volunteers that do not receive any treatment.