Ответ:

  1. \(20a^3(5a)^2=20a^3\cdot25a^2=500a^5\).
  2. \(-0.4x^5(2x^3)^4=-0.4x^5\cdot16x^{12}=-6.4x^{17}\).
  3. \((-c^3)^2\cdot12c^6=c^6\cdot12c^6=12c^{12}\).
  4. \((3x^5y^3)^4\cdot\left(-\frac1{81}xy^2\right)=81x^{20}y^{12}\cdot\left(-\frac1{81}xy^2\right)=-x^{21}y^{14}\).
  5. \(\left(-\frac23ab^5\right)^3\cdot18a^5b=-\frac8{27}a^3b^{15}\cdot18a^5b=-\frac{16}{3}a^8b^{16}\).