ObyJenny
Let x, y and z be the number of lawyers with licence in only each of the states respectively.
Let a be the number of lawyers with licence in all 3 states
x + a = 10. (1)
y + a = 11. (2)
Z + a = 13. (3)
x + y + z = 28 (4)
Therefore:
x = 10 - a. (5)
y = 11 - a. (6)
z = 13 - a. (7)
10-a+11-a+13-a =28
34 -2a = 28
2a = 6
a = 3
From eqn 7
z = 13 - 3
z = 10
Logical Way:
28 Lawyers but 10+11+13 = 34 counts.
It means, we have 6 counts extra coming from lawyers who are on a triple duty (double duty is forbidden as it is given that a lawyer either works at 1 firm OR all 3).
Those on triple duty are leaving 2 counts each extra....6/2 = 3
This means, there are 3 lawyers that are on triple duty.
So, each firm’s count minus 3 will give lawyer’s working only for their own firm, which is 13 - 3 = 10 in case of firm Z
Mathematical Way :
Let P be lawyers working for ONLY 1 firm
Let Q be lawyers working for all the 3 firms
Total count = 10+11+13 = 34
Number of lawyers = 28 (given)
So,
P*1 + Q*3 = 34 and,
P + Q = 28
This gives P as 25, and Q as 3.
Lawyers working only at a single firm would be firm size minus Q,
which, for Firm Z, is 13 - 3 = 10