B. S. Grewal Engineering Mathematics- Probability

Question

A student has to answer $9$ out of $12$ questions. How many ways if he has to answer at least $4$ out of first $5$ questions?

Comments

this question is not related to probability

Answer 1

At least 4 out of first 5 means 4, or 5 out of first 5 questions (so 5, and  4  question out of next 7).  

$ _{4}^{5}\textrm{C} *_{5}^{7}\textrm{C} + _{5}^{5}\textrm{C} * _{4}^{7}\textrm{C} $

$=5*21 + 1*35$

$= 105 + 35$

$=140$

Comments

Gr8, may be I have the old version.

Thanks for the solution

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ok

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140 will be correct as in the question it is mentioned that at least 4 out of first 5 have to be chosen. so there are two cases-either choose 4 out of first 5 or choose all first 5