Which number replaces the question mark?

Pituitary: Brain :: Thymus : ? 

Samuel is eighth shortest in a class of 26 students. What will be his position if counted from the tallest in the class?

Which on set of letter complete the given series ?
_bc_ca_aba_c_ca

In the following question, select  the related from the given alternatives.
Raise : Accession :: Famous :?

In each of the following questions, which one of the four interchanges in signs and numbers would make the given equation correct?
6x4+2=16 

Deepak said to Nitin, " That boy playing football is the younger of the two brothers of the daughter of my father's wife. " How is the boy playing football related to Deepak? 

If it was Friday on 3rd date of a month, then which day would come on the 4th day after 21st date of that month?