Try to ignore the 4 groups part, that is extraneous infromation. This is because each of the 4 groups are further divided n groups(n is unknown).
Initially you have s subjects, they are finally divided into n units with each unit has u subjects. Thus number of units n = s/u. Now each unit has 1 researcher. Thus number of units = number of researchers.
Thus number of researchers = s/u.
It's easy to say you can't do it , you are not good enough. It's much harder to step in your shoes and achieve the improbable.