This is a similar question from the book "Valued Fields by Antonio J. Engler and Alexander Prestel, Springer, 2005 " page 82, Exercise 3.5.4.(b).

Let $(K_{1}, V_{1})\subseteq (K_{2}, V_{2})$ be finite extension of valued fields. Assume that $[K_{2} : K_{1}] = n$. Let $G_{1}$ and $G_{2}$ be value groups of $V_{1}$ and $V_{2}$ respectively. Prove that $[G_{2} : G_{1}] = n$. Thanks