Chapter 13: Probability

Probability builds on Class 9's classical formula with the complementary-event relationship, P(E)+P(not E)=1, applied across coins, dice, cards, marbles, and even geometric probability based on area. Knowing P(drawing a red card)=1/2 immediately gives P(not red)=1/2 too, without any further counting.

Nearly every problem here reduces to one idea — count the favourable outcomes, count the total, divide — so the real skill is correctly identifying both counts for each newly-dressed scenario, not learning new formulas. Forgetting that a sample space shrinks after an item is removed without replacement is a frequent source of error, carried straight over from the same trap in Class 9.