Ответ:

1) \(\frac{a^3b}{15c}\cdot\left(-\frac{3c}{a^2b^2}\right)=-\frac{3a^3bc}{15ca^2b^2}=-\frac{a}{5b}\).

2) \(18y^3\cdot\frac{4x^2}{9y^5}=\frac{72x^2y^3}{9y^5}=\frac{8x^2}{y^2}\).

3) \(\frac{a^{2n}b^{n+4}c^{2n+3}}{c^{n+3}a^{2n}b^{n+1}}=\frac{c^n b^3}{1}=\frac{c^n}{b^3}\).

Ответ: 1) \(-\frac{a}{5b}\); 2) \(\frac{8x^2}{y^2}\); 3) \(\frac{c^n}{b^3}\).