Question 394009
Let b = probability that a boy is chosen, and 
g =  probability that a girl is chosen.

Then from the given,

b = 3g/4.
Also, b + g = 1, because the two probabilities must add up to 1.
By substitution, 

3g/4 + g = 1, or 7g/4 = 1, or g = 4/7, the probability that a girl is chosen.

Incidentally, this is also the fraction of girls in the debate team.