$\dfrac{5}{15}=\dfrac{1}{3}$. 
In an opaque bag, there are $5$ red balls, $5$ white balls, and $5$ yellow balls. The balls are the same except for their colours. Take out one ball from the bag. What is the probability that you take out a yellow ball? 
Options:
a) $\dfrac{1}{2}$ 
b) $\dfrac{1}{3}$ 
c) $\dfrac{1}{4}$ 