The ratio of male to female students in Yucai Primary School was 7:5, and then 12 girls were transferred. At this time, the ratio of boys to girls is 9:7. How many boys are there in the school?
The number of boys remains the same, and the unit for boys is 1:
Girls accounted for 5/7 in the original 1 unit, and 7/9 in the later 1 unit.
The difference before and after 12 girls, the unit of 1 divided by 12÷(7/9-5/7)= 189 girls.
A: School boys 189.