SOLUTION: A bag contains 6 red discs, 3 yellow discs and an unknown number of black discs. The probability of selecting a red disc is 2/5. How many black discs are in the bag. Thanks

Simple definition of probability = No. of possible  "favourable" outcomes / No. of total possible outcomes.



Let the bag have x black discs.



Then the total number of discs = 6 + 3 + x = x + 9



From this we have to select a red disc.



No. of red discs  = No. of favourable outcomes = 6

No. of total possible outcomes = x + 9



So, 6/(x + 9) = 2/5



Or, 3/(x + 9) = 1/5



Cross-multiplying, x + 9 = 15



So, x = 6



There are 6 black discs in the bag.



:)