Impossible event

No outcome favours an impossible event, so its count is 0 and its probability is 0. At the other end is a sure event, also called a certain event. Every outcome favours it, so its probability is 1. Every other event lies between these two. A probability can never be below 0 or above 1.

A small example

Throw one die. P(getting 7) = 0/6 = 0, because no face shows 7. That is an impossible event. P(getting a number less than 7) = 6/6 = 1, because every face counts. That is a sure event. An answer such as 7/6 is a sign the counting has gone wrong.

Where marks go

Question 19 of CBSE's 2026-27 Maths Basic sample paper is an assertion-reason question. The assertion says the chance of getting 8 on a die is 0. The reason says an impossible event has probability 0.

The 2026-27 Maths Basic marking scheme picks option (A): both are true, and the reason explains the assertion. It carries 1 mark, and Section A gives no part marks. So both statements, and the link between them, have to be judged right.

About the author

Mohit Sardana

Chief Architect

An engineer (B.Tech, Computer Science) with over 20 years of preparing students for entrance exams, including as Dean at FIITJEE and Vice-President at Aakash. He is the chief architect of the TRUpreBoards evaluation engine.

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