A box contains 7 red and 6 blue balls. Two balls are taken at random without replacement. Find the probability that they are of different colours.
Answer and explanation
Correct answer: D.
- Step 1: Red then blue: ; blue then red gives the same value. Step 2. . **Answer: . Why not the others:** is what you get if you count only red then blue. is what you get if you keep the total at for the second ball. is what you get if you give the probability that they are the same colour.
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