There's not enough information to find an answer.
I think the idea here is that in descending (416 - 278) = 138 meters,
the glider gives up some gravitational potential energy, which
becomes kinetic energy at the lower altitude. This is all well and
good, but we can't calculate the difference in potential energy
without knowing the mass of the glider.
Distance traveled by the ball is given by

here we know that
speed = 20 m/s
times = 0.25 s
now we have


so ball will travel 5 m distance in the given interval of time
1 gallon = 231 cubic inches
1 cubic foot = 1728 cubic inches
(55 gal) x (231 in³/gal) x (1 ft³/1728 in³)
= (55 x 231 / 1728) ft³
= 7.352 cubic feet (rounded)