ITPEC FE Morning October 2022 Question 6
ITPEC FE Morning October 2022 — Question 6 of 80
Probability without replacement — calculate probability of drawing 2 blue balls from a mixed box.
Given: 3 blue + 4 green = 7 total balls. Draw 2 without replacement.
- •P(1st blue) =
3/7 - •P(2nd blue | 1st blue) =
2/6=1/3 - •P(both blue) =
3/7 × 1/3= 1/7
Why not others:
- (a) 2/49 — incorrect numerator in probability calculation
- (b) 1/15 — wrong simplification
- (d) 9/49 — uses (3/7)², which assumes replacement (putting the ball back)
Key rule: Without replacement → reduce both numerator and denominator after each draw. With replacement → denominators stay the same.
AI-generated — may contain errors
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