2017 AMC 10A Problem 4

Note that after $30$ seconds, there are $3$ toys added and $2$ removed, leaving a net total of $+1$ toys in the box. We have to be careful towards the end, however, since it is possible for the box to have $30$ toys right after Mia’s mom adds the toys and before Mia removes them. After there are $27$ toys in the box, Mia’s mom can add $3,$ leaving $30$ toys in the box. It will take $27 \cdot 30$ seconds, plus another $30$ seconds, which gives us $$28 \cdot 30 \div 60 = 14$$ minutes to get $30$ toys in the box. Thus, B is the correct answer.