question from game theory

There are two types of workers: bad workers and good workers. Each worker has a utility function of the shape U=M-C, where M stands for annual income (measured in thousand of $), and C stands for the level of effort done (measured in weeks). A good worker can earn up to 121 thousand annually, while bad worker can earn only 64 thousand. The employer can only verify the type of worker by checking the number of certificates. If a good worker has to spend one week to get a certificate, and the bad worker has to spend 4 weeks, calculate reasonable number of certificates the employer can require.