Probability (Basic)

The classical probability formula, built directly on the counting techniques from Permutation and Combination.

📖 ~4 min read 📐 RRB NTPC / Group D — Mathematics

Core Formula

Probability of an event = (Number of favourable outcomes) / (Total number of possible outcomes)
Probability always lies between 0 and 1. P(not A) = 1 − P(A)

💡 Example: A bag has 5 red and 3 blue balls. Probability of drawing a blue ball = 3/8

💡 Example: Two dice are thrown. Probability that the sum is 8 → Favourable pairs: (2,6),(3,5),(4,4),(5,3),(6,2) = 5. Total = 36. Probability = 5/36

⚠️ For "at least one" problems, it's usually faster to find P(none) and subtract from 1, rather than adding every individual case.

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