`18)`
`(3/4)^3 : (3/4)^2 : (-2/3)^3`
`= (3/4)^(3-2) : ((-2)^3)/(3^3)`
`= 3/2^2 : ((-2)^3)/(3^3)`
`= 3/2^2 . (3^3)/((-2)^3)`
`= (3^4)/((-2)^5) = -81/32`
$$$$
`19) `
`3^2 . 1/243 . 81^2 . 1/3^3`
`= 3^2 . 1/(3^5) . (3^4)^2 . 1/3^3`
`= 3^2/3^5 . 3^8/3^3`
`= 1/3^3 . 3^5 = 3^5/3^3`
`= 3^2 = 9`
$$$$
`20)`
`( 4 . 2^5) : ( 2^3 . 1/16 )`
`= ( 2^2 . 2^5 ) : ( 2^3 . 1/2^4 )`
`= 2^7 : 2^3/2^4`
`= 2^7 : 1/2`
`= 2^7 . 2/1 = 2^8 `
`= 256`
