18. Find the product: ((2x^4 yz^2)/7)((7xy^2 z^4)/6)
Ans.
((2x^4 yz^2)/7)((7xy^2 z^4)/6)=(2×7×x^5 y^3 z^6)/(7×6)=(x^5 y^3 z^6)/3

19. Find the additive and multiplicative inverses of: 221/10
Ans.
Additive inverse,
221/10-221/10=0
And multiplicative inverse,
221/10×10/221=1

20. The cost to ship a package from Tulsa to Philadelphia is $7.40 per pound for the first 2 pounds and $3.85 per pound for each additional pound. Find the cost to ship a 10-lb package from Tulsa to Philadelphia.
Ans.
For 2 pounds,
2×$7.40=$14.80
For additional 8 pounds,
8×$3.85=$30.80
For 10 pounds,
$14.80+$30.80=$45.60