Probability is written as a fraction.
On the top (numerator) of the fraction, we place the number of options we are looking for.
On the bottom (denominator) of the fraction, we place the total number of possible combinations.
Look at these coloured cards.
What is the probability of selecting a yellow card at random?
There are 4 options that "we are looking for". This is the four yellow cards.
There are 6 "possible combinations".
The probability of choosing a yellow card is 4/6 which reduces to 2/3.