`a) 4/5-(-1/3)^3-|-4/5|-7/8`
`= 4/5 + 1/27 - 4/5 - 7/8`
`= (4/5-4/5)+(1/27-7/8)`
`= -181/216`
`b) (-7/9+3/11):3/4-(2/9-19/11):3/4`
`= (-7/9+3/11).4/3-(2/9-19/11).4/3`
`= 4/3(-7/9+3/11-2/9+19/11)`
`= 4/3(-1+1)`
`= 4/3. 0`
`= 0`
`c) (4^6. 6^4)/(3^3 . 8^6 . 15)`
`= [(2^2)^6. 2^4. 3^4]/[3^3.(2^3)^5. 3. ]`
`= (2^16. 3^4)/(3^4. 2^15)`
`= 2`