I assume that there are 360 days in a year.
Simple discount
25,000×0.13×(100÷360)=902.78
Cash in hand at the beginning of 100 days is
25,000−902.78=24,097.22
Effective rate of interest is
902. 78=24097.22×r×(100÷360)
Solve for r
r=902.78÷(24,097.22×(100÷360))
r=0.1349×100
r=13.49%.....answer