Answer
At the top of the cliff PE (potential energy) = M g H
Halfway down the cliff PE = M g H / 2 the potential energy is 1/2 of that at the top of the cliff
Since PE and KE are conserved
KE (halfway down the cliff) = M g H / 2
Apparently the problem giver assumes g = 10 m/s^2
If so M g H = 10 * 10 ^ 250 = 25,00 Joules
1/2 M g H =12,500 Joules potential energy 1/2 way down
KE = 1/2 M v^2 = 1/2 ^ 10 * 50^2 = 12,500 Joules
One must assume a value of 10 m/s^2 for g if g is to be eliminated from the result, otherwise the results would contain g.