Let f be a real valued continuous function defined on the positive real axis such that $\mathrm{g}\left(\mathrm{x}\right)={\int }_0^{\mathrm{x}} \mathrm{t}f\left(\mathrm{t}\right)\mathrm{dt}.$ If $g\left(x^3\right)=x^6+x^7\text{, then value of }\sum _{r=1}^{15} f\left(r^3\right)\text{ is: }$