Question

Jillian guessed at random for each question on a true-false quiz of ten questions. what is the probability that she got exactly seven questions correct?​

Answer 1

11/64

Explanation:

Each question can be answered in 2 ways

∴ Total ways in which 10 questions can be answered

=2¹⁰

No. of ways of guessing at least 7 questions

out of 10

=7 questions correct +8 questions correct

+9 questions correct +10 questions correct

= ¹⁰C7(1)⁷+ ¹⁰C8(1)⁸+ ¹⁰C9(1)⁹+ ¹⁰C10(1)¹⁰

= 120 + 45 + 10 +1

=176

..Required Probability =176/2¹⁰

.. =11×16/2¹⁰

=11/64