If the sum of the squares of the deviations of ranks of two students in seven subjects is 21. then the coefficient of rank correlation is:

here we have 

number of subject n = 7 

square of sum of deviation are $$ \sum_di^2$$= 21 

now we know the fourmula of the coefficient of rank correlation  is rR = 1- $$\times {n-1}\sum_di^2\frac\times{n}{n^2-1}$$ 

       the coefficient of rank correlation is rR = 1- $$\times {7-1}\sum_21 \frac\times{7}{7^2-1}$$

                                                               rR = 1- $$\frac{3}{8}$$

                                                               rR=  $$\frac {5}{8}$$ answer