Solution
For this case we have the following info:
Class 1: Total 18 students, 8 girls and 10 boys
Class 2: Total 17 students, 10 girls and 7 boys
The total of students are: 18+17 = 35
Total of boys: 10 + 7 = 17
Then the ratio of boys to the total number of students is given by:
r = 17/35