Dear Colleagues

I have a dataset with the following shape:

var1.......var5......var10(which sum obs in var11)... var11
co. N...........1203.........................3................................
.......................................................................1450
........................................................................1203
........................................................................1620
co.N......etc

so, I need to drop the observations in var11 which equal the unique value in var5. in the example above I want to drop the observations in var11=1203 that =1203 in var5

I tries
drop var11 if (var11=var5) but doesn't work and give me var11=var5 invalid name
or type mismatch if I put ==

in addition how can I change the value of var10 to be 2 in the example above instead of 3 after dropping the observation that =1203 in var11.

Thanks in advanced