What does this mean

Station A is located at $3+25\mathrm{.}41$ with an elevation of $465\mathrm{.}88ft.$ Station B is located at $6+21\mathrm{.}19.$ Station $C$ is located at $18+14\mathrm{.}26.AB$ has a grade of $+2\mathrm{.}8\%$ and $BC$ has a grade of $-6\mathrm{.}7\%.$ What is the elevation of $15+00(ft)?$

$421\mathrm{.}98$

$415\mathrm{.}28$

$408\mathrm{.}58$

$401\mathrm{.}88$