Elementary event

Some events have one outcome. Others group several outcomes together. "Getting a 4" on a die is elementary. "Getting an even number" is not, because 2, 4 and 6 all count. Add the probabilities of every single outcome of one experiment and you always get 1.

A small example

Toss two coins. The elementary events are HH, HT, TH and TT, each with probability 1/4. Their sum is 4 × 1/4 = 1. The event "exactly one head" is made of two of them, HT and TH. So its probability is 2/4 = 1/2.

Where marks go

With two dice, each pair such as (2, 3) is one elementary event, and there are 36 pairs. Question 13 of CBSE's 2026-27 Maths Standard sample paper asks for a number less than 4 on each die. The 2026-27 Maths Standard marking scheme lists the 9 pairs that work, giving 9/36 = 1/4.

Question 13 is multiple-choice, with no part marks. In Question 25 of the same paper, listing the favourable pairs earns half a mark on its own under step marking. Writing the pairs out also shows that (2, 3) and (3, 2) are two different outcomes. For how probability is worked out from these counts, see theoretical probability.

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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