Explanation
Add the number of marbles to get the total number in the bag; 12 + 3 + 6 + 4 = 25. Therefore, 25 is the number of possible outcomes. Seven marbles are either blue or yellow. Seven is the number of favorable outcomes;\(\frac{7}{ 25} \times \frac{4 }{4}=\frac {28}{100}=28 \%\)