`a) 3/11. 7/12 + 15/38 + 3/11. (-19)/12 + 23/38`
`= (3/11. 7/12 + 3/11. (-19)/12) + (15/38 + 23/38)`
`= 3/11. (7/12 + (-19)/12) + 1`
`= 3/11. (-1) + 1`
`= (-3)/11 + 11/11`
`= 8/11`
`b) (-0,25). 4/17. (-3 5/21). ((-7)/23)`
`= (-1)/4. 4/17. (-68)/21. ((-7)/23)`
`= ((-1).(4).(-68).(-7))/(4.17.21.23)`
`= -4/69`
`c) ( (-3)^10. 15^5)/(25^3. (-9)^7)`
`= ( 3^10. 3^5. 5^5)/( (5^2)^3. [-(3^2)^7] )`
`= (3^15. 5^5)/(5^6. (- 3^14)`
`= (-3)/5`
`d) (2/5 sqrt {16} + 2 sqrt {16/25}) : 2 sqrt {1/16}`
`= (2/5. 4 + 2. 4/5) : 2. 1/4`
`= (8/5 + 8/5) : (2 : 1/4)`
`= 16/5 : 8`
`= 2/5`
