Ответ:

  1. \(20a^3\cdot(5a)^2=20a^3\cdot25a^2=500a^5\).
  2. \(-0.4x^5\cdot(2x^3)^4=-0.4x^5\cdot16x^{12}=-6.4x^{17}\).
  3. \((-c^3)^2\cdot12c^6=c^6\cdot12c^6=12c^{12}\).
  4. \((3x^6y^3)^4\cdot\left(-\frac{1}{81}xy^2\right)=81x^{24}y^{12}\cdot\left(-\frac{1}{81}xy^2\right)=-x^{25}y^{14}\).
  5. \(\left(-\frac{2}{3}ab^3\right)^3\cdot18a^5b=-\frac{8}{27}a^3b^9\cdot18a^5b=-\frac{16}{3}a^8b^{10}\).

Ответ: 1) \(500a^5\); 2) \(-6.4x^{17}\); 3) \(12c^{12}\); 4) \(-x^{25}y^{14}\); 5) \(-\frac{16}{3}a^8b^{10}\).