jmc

In bag $A$, there are $26$ jellybeans, and $50\%$ are yellow. That means there are $26\times 50\%=26\times 0.50=13$ yellow jelly beans in this bag. In bag $B$, there are $28$ jellybeans, and $25\%$ are yellow. That means there are $28\times 25\%=28\times 0.25=7$ yellow jelly beans in this bag. In bag $C$, there are $30$ jellybeans, and $20\%$ are yellow. That means there are $30\times 20\%=30\times 0.20=6$ yellow jelly beans in this bag. In all three bags, there are $13+7+6=26$ yellow jelly beans in total , and $26+28+30=84$ jelly beans of all types in total. Thus, $\frac{26}{84}=\frac{26}{84}\cdot 100\%=\frac{13}{42}\cdot 100\%=30.9\%$ of all jellybeans are yellow. Thus, the correct answer is $\boxed{31\%}$