Total number of possibilities to answer the 150 questions: 4^150
Possibilities when 0 good answer: 3^150 // 3 possibilities for each question in order to be always wrong
Possibilities when 1 good answer: (3^149)*150 // 3 wrong possibilities for 149 questions and only 1 possibility to get the right one // The right solution can either be the first, the sedond or the 150th question, so we multiply by 150.
Possibilities when 2 good answers or more than 2:
4^150 - ( 3^150 + (3^149)*150)= 4^150 - ( 3^150*(1+50) )
=4^150 - 51 * 3^150
4^150 - 51 * 3^150 is the total number of ways in which a student can answer atleast 2 out of 150 questions
Last edited by
pepeprepa on Thu Jul 24, 2008 6:54 am, edited 1 time in total.
