The following given problem can be expressed in the form of:
 E = 5.60 * T1 + 8.40 * T2
  
 where,
 E = total earning
 T1 = normal hours worked
 T2 = overtime hours
  
 Assuming that the maximum T1 hours is 40 hours. Beyond that is considered overtime, therefore:
  
 287 = 5.60 * 40 + 8.40 * T2
 T2 = 7.5 hours
  
 Therefore 7.50 overtime hours is needed.