The flexure formula
was derived for a homogeneous beam, i.e., E is constant for the entire cross section. The reinforced timber beam to be analyzed here is not homogeneous — it consists of two different materials having differing elastic moduli.
Although the flexure formula cannot be applied directly, we can transform the cross section so that the flexure formula is applicable. For calculation purposes, we will transform one of the materials into an equivalent amount of the other material.
For this problem, the CFRP will be transformed into an equivalent amount of the Douglas fir material.