Explanation:

- The problem states that 30% of $a=b$.

- We want to find out what $b\mathrm{\%}$ of 50 is equivalent to.

- Since $b=0.3a$, we can substitute $b$ in the expression.

- Calculate $b\mathrm{\%}$ of 50: $(b/100)\times 50=(0.3a/100)\times 50=0.15a$.

- Option 3: 15% of a is indeed equivalent to 0.15a.

- Therefore, $b\mathrm{\%}$ of 50 equals 15% of a.

Option 3: 15% of a is the correct answer.