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another probability question

Expert replies
Source: — Problem Solving |

by clock60 » Wed May 12, 2010 12:04 pm
Deljef18 wrote:A die is rolled twenty times and the number of twos that come up is tallied. Find the probability of getting exactly five twos.

Answer is 0.129.
i am not sure that it is gmat problem
but to me the answer looks like

(5/6)^15*(1/6)^5*(20!/(15!*5!))
if my reasoning is correct it is impossible to solve with time constrains
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