These scores:

student - score 
a - 70 
b - 70 
c - 70 
d - 60 
f - 60 
e - 50
[download]

simply must lead to following ranking:

Students Rank:
1. a,b,c
4. d,f
6. e
[download]

Ranking the score-values is of little practical use.
As Abigail mentionned above: 50 is the 3rd Score (if you rank scores without considering their actual occurrence).
But student e with score 50 was last of 6.