For every girl taking classes at the martial arts school, there are 3 boys who are taking classes at the school. if there are 236 students taking classes, write and solve a proportion to predict the number of boys taking classes at the school.

G = number of girls taking the martial arts classes
and
B = number of boys taking the martial arts classes

We know that for every girl, there are 3 boys.  Therefore, the proportion of girls to boys is:
= 

When you simplify this ratio by cross-multiplying you get: 
3G = B
When you think about this, it makes sense because you know that there are 3 times as many boys as there are girls.  Therefore you would need to multiply the number of girls by 3 to equal the number of boys in the class.

We know that there are 236 total people in the class, Therefore we know that:
G + B = 236

You can replace the B in the equation above with your simplified ratio to solve, since both contain the variable B:

G + B = 236
is the same as
G + 3G = 236 ->
4G = 236 ->
G = 59

So there are 59 girls in the class, but you need to figure out the number of boys, not girls.  Therefore, subtract 59 from 236 to get to the final

G + B = 236
59 + B = 236
59 + B - 59 = 236 - 59
B = 177