Labor unions can use the power of collective bargaining to negotiate, meaning that they will

Meaning that they (the labor unions) will use numbers to assert their power to strike

Collective bargaining refer to the negotiation process between the unions and the employee to negotiate the worker's salary. The more members the union have the more the company feels pressured to subject to their demand since it stunt the progress of their daily operational activities

I’m pretty sure it’s 4, you can check on the calculator