Test preparation

Question

A class is given 20 study problems from which 10 will be part of the exam. Student knows how to solve 15 of them. What are the chances that he will correctly answer:

a) all 10

b) exactly 8

c) at least 9

Answer 1

The definition of probability of an event is the key to answering this question:

P (A) = the ways that A can occur / Number of total outcomes

Since the student knows how to solve 15 of 20 problems, then there are 5 that the student does not know the answer.

a) In order for 10 correct answers to be chosen on the test means that 0 of the 5 unknown questions NOT be chosen therefore the answer is 1 - 5/20 = 75% or 15/20 = 75%

b)Exactly 8 means that 3/5 unknown would NOT be chosen 1-3/20 = 85%

c) 1-4/20 = 80%