if the sum of the first 15 terms of the seies ${\left(\frac{3}{4}\right)}^3+{\left(1\frac{1}{2}\right)}^3+{\left(2\frac{1}{2}\right)}^3+3^3+{\left(3\frac{3}{4}\right)}^3+....$s equal to 225 k, then k is equal to: