3. 计算:
(1)$-20+3+5-7$; (2)$-23+18-1-15+23$;
(3)$-7.2-0.8-5.6+11.6$; (4)$\frac{1}{6}-\frac{5}{6}-\frac{2}{3}$.

解: 原式=(-20-7)+(3+5)
         \ \ \ \ \ \ \ =-27+8
        \ \ \ \ \ \ \ =-19
解：原式=(−23−1−15)+(18+23)
   \ \ \ \ \ \ \ \ \ \ \ \ \ =-39+41
   \ \ \ \ \ \ \ \ \ \ \ \ \ \ =2
解：原式=(-7.2-0.8-5.6)+11.6
        \ \ \ \ \ \ \ \ =-13.6+11.6
        \ \ \ \ \ \ \ \ =-2
解：原式$=\ \frac{1}6-(\frac{5}6+\frac{2}3)$
$         \ \ \ \ \ \ \ \ =\ \frac{1}6-\frac{9}6$
$         \ \ \ \ \ \ \ \ =\ -\frac{4}3$

4. 计算:
(1)$12-(-18)+(-7)-15$; (2)$-0.6+2.4-(+0.4)+(-1.4)$;
(3)$\frac{1}{2}+(-1)-\left(-\frac{3}{4}\right)$; (4)$(-0.25)+(-3)-\left|-1\frac{3}{4}\right|-(-3)$.

解：原式=12+18+(-7-15)
        \ \ \ \ \ \ \ \ =30+(-22)
        \ \ \ \ \ \ \ \ =8
解：原式=-0.6+2.4-0.4-1.4
        \ \ \ \ \ \ \ \ =(-0.6-0.4-1.4)+2.4
        \ \ \ \ \ \ \ \ =-2.4+2.4
        \ \ \ \ \ \ \ \ =0
解：原式$=\ \frac{1}2-1+\frac{3}4$
$         \ \ \ \ \ \ \ \ =\ \frac{1}2+\frac{3}4-1$
$         \ \ \ \ \ \ \ \ =\ \frac{5}4-1$
$         \ \ \ \ \ \ \ \ =\ \frac{1}4$
解：原式$=-0.25-3+1\frac{3}4+3$
$        \ \ \ \ \ \ \ \ =(-0.25-3-\ 1\frac{3}4)+3$
        \ \ \ \ \ \ \ \ =-5+3
        \ \ \ \ \ \ \ \ =-2