There are 12 employees , who are divided into 3 groups , A,B AND C. Salary of each in A, B and C are 9000, 8000 and 4000 respectively. Total salary paid to all employees is 79000. Find no of employees in B

Let number of employees in A,B,C be a,b,c resp.
Then,
a+b+c=12 -----------------------Eq 1

9000a+8000b+4000c=79000 => 9a+8b+4c=79-----------------Eq. 2

From eq. 2, we can say that a has to be an odd number so that 9a+8b+4c can be odd(RHS is odd)
Possible values of 'a' can be 1,3,5,7. (as a cannot be more than 8)

Now putting each value of 'a' in the two equations you can get a value of 'b' and 'c'.
On calculating for a=3,
you can get value of b=4.