Yes, your answer checks out. I worked it out in a similar way and got 16.51m by subtracting the cotangents of the angles multiplied by 50. Since you worked out the answer yourself I've deducted the 40 points I received for this answer, since there's no facility for commenting on an answer provided in a question! I've taken the height differences of all three people to be negligible in comparison with the height of the hill, but if the angles of depression are measured from the top of the hill by an instrument lying flat on the ground and with the boys' feet in view rather than their heads, the heights don't come into it!