Assume
winning means a simple majority or over half of the votes.
If it takes
at least half voting Yes to pass a motion, the motion
did not pass. 
There are different ways to solve the
problem.
Look at different numbers of boys and girls. Think about
each of the following:

A class of 50 boys and 50 girls

A class of 0 boys and 100 girls

A class of 100 boys and 0 girls
In neither of the extreme cases nor in any other case
will the motion pass. 
Above the line, you see that 100% of
the boys are depicted to the left of the bold line [light
blue rectangle] with 100% of the girls to the right [pink
rectangle]. Below the line are the voting percentages
of Yes and No for both boys and girls.
To determine if the Yes vote wins, then the segments
representing the Yes votes together have to be over half
the length of the entire segment. The segments representing
Yes will never add to half of the total length so the
motion cannot pass. 