Explanation:
this creates 2 similar triangles.
that means all angles are the same. and all the side lengths of one triangle correlate to the corresponding side lengths of the other triangle by the same multiplication factor.
her shadow is 23.05 - 18.9 = 4.15 m long.
the factor between the shadow lengths is then
4.15 × f = 23.05
f = 23.05 / 4.15 = 5.554216867...
the same factor now applies to the relation of the heights :
1.45 × 5.554216867... = 8.053614458... m
rounded the tree is 8.05 m tall.