This is easily set up incorrectly (as I just did and restarted).

The important thing is that the separate walk times remain fixed.

From the first statement

$4 t_1 + 5 t_2 = 35$ and from the second

$5 t_1 + 4 t_2 = 37$

or

$\begin{pmatrix}4 &5 \\ 5 & 4\end{pmatrix}\begin{pmatrix}t_1 \\ t_2\end{pmatrix} = \begin{pmatrix} 35 \\ 37\end{pmatrix}$