ANSWER:

If you are certain that there is a najasah but you don’t know where, some scholars say it is sufficient to wash the area most likely you believe the najasah is at. Other scholars say that you need to wash the whole trousers and I am inclined to this opinion. 

QUESTION: Is it permissible if I do not know where the impurity is, but know the trousers are impure to just clean where the impurity has ‘probably’ befallen.