To find a missing length with all other lengths given, you simply find their ratio. With this problem:
82.8m / 92m = 9/10
73.8m / 82m = 9/10
So, using this 9/10 size ratio, we just find what is 9/10 of 47m (which is 42.3) by doing 47(9/10). Boom, there's your answer.
