contestada

There is a bag filled with 4 blue and 5 red marbles.
A marble is taken at random from the bag, the colour is noted and then it is not replaced.
Another marble is taken at random.
What is the probability of getting 2 that are different in colour?

Respuesta :

1) first pick red 5/9 blue 4/9


2) second pick when red has been picked red now becomes 4/8, blue stays 4/8


2) Second pick when blue has been picked blue now becomes 3/8, red stays 5/8


Mind you, you only need both different colours so:

first pick red 5/9 x sec pick blue 4/8 = 20/72

and first pick blue 4/9 x sec pick red 5/8 = 20/72

add both up 20/72 + 20/72 = 40/72 = 5/9

Otras preguntas