The given expression is
[tex]\log _5(15x+20y)[/tex]Factor out the greatest common factor between 15x and 20y.
[tex]\log _5(5(3x+4y))[/tex]Then, use the product property to expand the logarithm.
[tex]\log _55+\log _5(3x+4y)[/tex]At last, simplify the first logarithm because its base is equal to its argument.
[tex]1+\log _5(3x+4y)[/tex]Therefore, the simplest form of the logarithm is
[tex]1+\log _5(3x+4y)[/tex]