Help please.(do all pls)

16. 5x^3 y^-5 • 4xy^3
20x^4y^-2

20x^4 • 1/y^2

=20x^4/y^2

17. -2b^3c • 4b^2c^2
= -8b^5c^3

18. a^3n^7 / an^4 (a^3 minus a = a^2 same as n^7 minus n^4 = n^3)
=a^2n^3

19. -yz^5 / y^2z^3

= -z^2/y

20. -7x^5y^5z^4 / 21x^7y^5z^2 (divide -7 to 21 and minus xyz)

= -z / 3x^2

21. 9a^7b^5x^5 / 18a^5b^9c^3

=a^2c^2 / 2b^4

22. (n^5)^4

n ^5 x 4
=n^20

23. (z^3)^6
z ^3 x 6
=z^18