Should he take his cue from the person next to him if he is unsure about the number of rak’ahs?
Question

                It so happens that when I join the prayer late, I forget how many rak’ahs had been done when I joined and how many are left for me to make up, and when I am sure that three or more people have come after me and joined the row with me at the same time… My question is: is it permissible to follow their example with regard to the remaining rak’ahs, especially since it is unlikely that a number of people would forget in the same manner?
Praise be to Allah.
    In the case of doubt as to the number of rak’ahs, one should 
    act on the basis of what is certain, so he should assume the smaller number 
    of rak’ahs. So if he acts according to what is more likely, i.e., what he 
    thinks is most likely to be the case, even by looking at those who are next 
    to him, there is nothing wrong with that, provided that he does sujood 
    al-sahw (prostration of forgetfulness) in either case.