Let the number of workers = x
Given that average salary of all the workers = Rs.8000
then total salary of all workers = 8000x
Given that average salary of 7 technicians is Rs.12000
=> Total salary of 7 technicians = 7×12000=84000
Number of the rest of the employees = (x - 7)
Average salary of the rest of the employees = Rs.6000
Total salary of the rest of the employees = (x - 7)(6000)
8000x = 84000 + (x - 7)(6000)
=> 8x = 84 + (x - 7)(6)
=> 8x = 84 + 6x - 42
=> 2x = 42

=> x = 21