Solution
Using cosine rule to solve for the length of the tunnel:
[tex]a^2=b^2+c^2-2bccosA[/tex]
c=length of side c
a=length of side a
b=length of side b
A=angle opposite c
[tex]\begin{gathered} a^2=116^2+247^2-2(116)(247)cos78^0 \\ a^=\sqrt{116^2+247^2-57304cos78} \\ a=250.1016 \end{gathered}[/tex]
Therefore the length of the tunnel = 250.1m
