FIND_IN_SET on part of the string

[pelegk2]

i have for example a string which represnsts departments:

5,6,7,11,16,18,

and i want to search at all the user if some 1 of them has on of this departments
for example user xxx may have 5,16

the thing is when i try it i recive nothing!
is there a way the override tis problem?