Answer:
N =50
Explanation:
In this case we have to write the two equations they give us and try to solve them
  N = P +10
They say 2.5P is twice the distance between the armpits
2.5P = 2N
We see that they are two equations with two unknowns, so they can be solved substitute in the first
 N = (2 / 2.5) N + 10
 N [1 - 2 / 2.5] = 10
 N [1 - 0.8] = 10
 N = 10 / 0.2
 N = 50 cm