A box has 5 red, 3 blue, 2 green balls. Probability of drawing a non-green ball?
- (a)0.5
- (b)0.8
- (c)0.6
- (d)0.7
Answer
Why
Correct — B. Count the balls the question wants, then divide by all the balls.
Rule: probability = favourable outcomes ÷ total outcomes.
Total balls: 5 + 3 + 2 = 10
Non-green balls: 5 red + 3 blue = 8
P(non-green) = 8 ÷ 10 = 0.8
Check by complement: 1 − 2⁄10 = 1 − 0.2 = 0.8. That is option (b).
Why the others are wrong
- (a)0.5 — 0.5 is 5 ÷ 10, the chance of a red ball. Non-green also takes in the 3 blue balls, so the count is 8, not 5.
- (c)0.6 — 0.6 means 6 favourable balls out of 10. No group of these colours makes 6: the possible counts are 2, 3, 5, 7, 8 and 10.
- (d)0.7 — 0.7 is 7 ÷ 10, which is 1 − 3⁄10: the chance of not drawing blue. The question leaves out the 2 green balls, not the 3 blue ones.
Concept
Probability compares the outcomes you want with all the equally likely outcomes. Each ball is one outcome, so the total here is 10.
Non-green is a complement: every ball except the green ones. Count it directly (red + blue = 8) or subtract the green share from 1 (1 − 0.2). The two routes must agree, so the second is a free check on the first.
The stem does not say 'at random' or 'one ball'. The key takes the standard reading: one ball is drawn, and every ball is equally likely.
Key facts
- Probability = favourable outcomes ÷ total outcomes.
- P(not A) = 1 − P(A).
- With 2 green balls among 10, P(green) = 0.2 and P(non-green) = 0.8.
Study next
Common traps
- Leaving out the wrong colour: removing the 3 blue balls instead of the 2 green gives 0.7.
- Counting the red balls alone as non-green, which gives 0.5.
Here a one-draw probability is set in the Reasoning section, stated in a single line, with four answer choices one tenth apart.
Related PYQs
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