Hi there, you need a few more parameters to calculate that. Basically it is the ratio between the charged hours and the salaried hours. A consultant also needs time for continuous education, trainings, interviewing candidates, holidays. Usually it is around 80%.
In this example, I think they wanted you to calculate the break-even point. If you have a consultant in France, starting level making 40kEUR let us just say, that means 167 EUR/day, let's assume 250 EUR with social expenses and miscellaneous.
The contribution margin for each day on project is 15%*3500EUR=525 EUR
Therefore if he is active at least 250/525=48% of the time, then the cabinet is not overstaffed. The staffing for the whole company is the weighted average of staffing rates.