Answer: 30
Explanation:
Think of each number in the ratio of 2:5 as a group. Each group has to be the same. 2 groups of boys : 5 groups of girls. There are 12 boys on the bus. So the 12 boys are split into 2 groups. 12/2=6. 6 boys are in each group. Since we now know that each group had 6 kids we just need to find the number of girls. There are 5 groups of 6 girls. 5x6=30. There are 30 girls on the bus.