If his solution doesn't work, try putting the answer checker for the popup immediately after that part of the question. i.e.:
...
d.) Now that we have 3 pieces of information in our vvaxt we can now determine the
remaining pieces. Since we really want to know how far the
bike will travel in the time it takes for the balloon to fall what
we need to calculate is the time (t) that it takes the balloon to
fall to the ground. Which formula will you use to solve for
time?
A.) [: V_f=v_o+at :]
B.) [:v_f^2=v_o^2+2ax:]
C.) [:x=v_o*t = (1/2*a*t^2):]
[@ $popup->menu() @]*
END_PGML
ANS( $popup->cmp() );
BEGIN_PGML
e.) How long will it take the balloon to fall to the ground?
[_____]{$tu}.
...