ANSWER
P(gold marble) = 1/12
EXPLANATION
In total, there are:
[tex]3+10+23=36[/tex]36 marbles in the bag, where only 3 are gold marbles.
The probability is:
[tex]P(\text{event)}=\frac{\#\text{times the event can happen}}{\#\text{posible outcomes}}[/tex]In this case, the number of posible outcomes is 36, because there are 36 marbles in the bag. The number of times the event can happen is 3, because there are 3 gold marbles:
[tex]P(\text{gold marble)}=\frac{3}{36}=\frac{1}{12}[/tex]