Final answer:
To find the number of boys in a group where the ratio of girls to total students is 2:9 and there are 21 girls, divide the number of girls by 2 to find one part of the ratio, multiply by 9 for the total students, and subtract the number of girls from the total.
Step-by-step explanation:
The student's question involves determining the number of boys in a group given the ratio of girls to the total number of students and the number of girls. The ratio is given as 2:9, meaning 2 parts out of 9 total parts represent the girls. To find the number of boys, we first find the value of one part by dividing the number of girls (21) by the girl's ratio number (2), which gives us 10.5. This value represents the count per single ratio unit. Since the total ratio units for the group is 9, we multiply 10.5 by 9 to find the total number of students, which is 94.5, but since we cannot have half a student, it rounds to 94. We subtract the number of girls from the total to get the number of boys, which is 94 - 21 = 73 boys.