There are $12$ balls in a box: $b$ blue, $y$ yellow and $3$ red balls. Three balls are randomly chosen. If the probability of choosing one blue, one yellow and one red is $3/11$, find the number of yellow balls in a box.
If $t$ is the total number of balls in a box, then: $t=3+b+y$
$A$: "We choose exactly one blue, yellow and red ball."
Substituting $t$ gives
Now we have one equation with two unknowns, so we need to define another event.
Because we already know $P(A)$, that new event should be the complementary event of $P(A)$.
How to define that event and how to evaluate it?