Theoretical probability
- CBSE · NCERT Class 10, Ch 14 Probability
The theoretical probability of an event is the number of outcomes that favour it divided by the number of all possible outcomes, when every outcome is equally likely.
Probability measures how likely something is, from 0 to 1. The theoretical way does not need an experiment. You count. First, count every possible outcome. Then count the ones that give the event you want. Divide the second count by the first. This works only when every outcome has the same chance.
A small example
A bag has 3 red balls and 5 blue balls. One ball is taken without looking. There are 8 possible outcomes, and 3 of them are red. So P(red) = 3/8.
Where marks go
Question 25 of CBSE's 2026-27 Maths Standard sample paper throws two dice. It asks for the chance that the product is a prime number. The 2026-27 Maths Standard marking scheme gives half a mark for the total of 36 outcomes. Listing the 6 that work earns another half. The answer, 1/6, earns 1 mark.
The total can change inside a question. In the same paper, Question 18 removes 6 cards from a pack, so 46 remain, not 52. Our analysis of the Maths Basic 2026 paper notes the same thing in Q31: the second answer uses 300 pens, not 200.
Sources
Facts last checked against these sources on 30 September 2026.
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