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Why does this work:

Solve[Probability[x <= t, x \[Distributed] NormalDistribution[]] == 0.95 ,t]

But this doesn't:

Solve[Probability[x <= t,x \[Distributed] StudentTDistribution[2.964, 2.071, 5]] ==0.95,t]

How can I solve this problem?

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Solve is an exact symbolic solver ... it works mostly with linear equations and polynomials and special cases like the Erf function which it knows how to invert. Since your problem is numerical anyway, try NSolve instead of Solve. –  wolfies Nov 15 '13 at 13:10
    
Same problem with NSolve. Doesn't work. –  CCommander Nov 15 '13 at 13:28
    
Works for me .... what version of Mma are you using? –  wolfies Nov 15 '13 at 14:16
    
Works for me too, but try FindRoot when NSolve doesn't work. FindRoot won't find all roots, just one, but it can handle any "black box" function. –  Szabolcs Nov 15 '13 at 23:52

1 Answer 1

(This may be better as a comment)

This may be an exercise in testing the use of NSolve (which does produce the answer [Version 9]) the normal distribution and StudentTDistribution can be solved with Quantile or InverseCDF, as can be verified by testing solutions.

Quantile[StudentTDistribution[2.964, 2.071, 5], 0.95]

or

InverseCDF[StudentTDistribution[2.964, 2.071, 5], 0.95]

both yielding:

7.13717

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Thank you very much! That helped a lot! –  CCommander Nov 22 '13 at 10:21

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