13. 直接写出得数。
$\frac{3}{8}+\frac{1}{8}=$ $\frac{5}{9}-\frac{2}{9}=$ $\frac{5}{6}-\frac{5}{6}=$ $\frac{1}{3}+\frac{1}{2}=$ $1-\frac{5}{8}=$ $\frac{7}{10}+\frac{5}{10}=$
$\frac{3}{8}+\frac{1}{8}=$ $\frac{5}{9}-\frac{2}{9}=$ $\frac{5}{6}-\frac{5}{6}=$ $\frac{1}{3}+\frac{1}{2}=$ $1-\frac{5}{8}=$ $\frac{7}{10}+\frac{5}{10}=$
答案：$\frac{1}{2}$，$\frac{1}{3}$，0，$\frac{5}{6}$，$\frac{3}{8}$，$\frac{6}{5}$
解析：$\frac{3}{8}+\frac{1}{8}=\frac{4}{8}=\frac{1}{2}$；$\frac{5}{9}-\frac{2}{9}=\frac{3}{9}=\frac{1}{3}$；$\frac{5}{6}-\frac{5}{6}=0$；$\frac{1}{3}+\frac{1}{2}=\frac{2}{6}+\frac{3}{6}=\frac{5}{6}$；$1-\frac{5}{8}=\frac{3}{8}$；$\frac{7}{10}+\frac{5}{10}=\frac{12}{10}=\frac{6}{5}$
解析：$\frac{3}{8}+\frac{1}{8}=\frac{4}{8}=\frac{1}{2}$；$\frac{5}{9}-\frac{2}{9}=\frac{3}{9}=\frac{1}{3}$；$\frac{5}{6}-\frac{5}{6}=0$；$\frac{1}{3}+\frac{1}{2}=\frac{2}{6}+\frac{3}{6}=\frac{5}{6}$；$1-\frac{5}{8}=\frac{3}{8}$；$\frac{7}{10}+\frac{5}{10}=\frac{12}{10}=\frac{6}{5}$
14. $\frac{2}{7}+\frac{3}{8}+\frac{5}{8}$ $\frac{3}{7}+\frac{5}{6}+\frac{4}{7}$ $\frac{2}{7}+(\frac{3}{8}+\frac{5}{8})$ $(\frac{3}{7}+\frac{4}{7})+\frac{5}{6}$
答案：$\frac{9}{7}$，$\frac{11}{6}$，$\frac{9}{7}$，$\frac{11}{6}$
解析：$\frac{2}{7}+\frac{3}{8}+\frac{5}{8}=\frac{2}{7}+(\frac{3}{8}+\frac{5}{8})=\frac{2}{7}+1=\frac{9}{7}$；$\frac{3}{7}+\frac{5}{6}+\frac{4}{7}=(\frac{3}{7}+\frac{4}{7})+\frac{5}{6}=1+\frac{5}{6}=\frac{11}{6}$；$\frac{2}{7}+(\frac{3}{8}+\frac{5}{8})=\frac{2}{7}+1=\frac{9}{7}$；$(\frac{3}{7}+\frac{4}{7})+\frac{5}{6}=1+\frac{5}{6}=\frac{11}{6}$。发现：整数加法的交换律和结合律对分数加法同样适用。
解析：$\frac{2}{7}+\frac{3}{8}+\frac{5}{8}=\frac{2}{7}+(\frac{3}{8}+\frac{5}{8})=\frac{2}{7}+1=\frac{9}{7}$；$\frac{3}{7}+\frac{5}{6}+\frac{4}{7}=(\frac{3}{7}+\frac{4}{7})+\frac{5}{6}=1+\frac{5}{6}=\frac{11}{6}$；$\frac{2}{7}+(\frac{3}{8}+\frac{5}{8})=\frac{2}{7}+1=\frac{9}{7}$；$(\frac{3}{7}+\frac{4}{7})+\frac{5}{6}=1+\frac{5}{6}=\frac{11}{6}$。发现：整数加法的交换律和结合律对分数加法同样适用。
15. $\frac{5}{8}-(\frac{3}{8}+\frac{1}{12})$ $\frac{2}{3}-\frac{1}{4}-\frac{1}{4}$ $\frac{5}{8}-\frac{3}{8}-\frac{1}{12}$ $\frac{2}{3}-(\frac{1}{4}+\frac{1}{4})$
答案：$\frac{1}{6}$，$\frac{1}{6}$，$\frac{1}{6}$，$\frac{1}{6}$
解析：$\frac{5}{8}-(\frac{3}{8}+\frac{1}{12})=\frac{5}{8}-\frac{3}{8}-\frac{1}{12}=\frac{2}{8}-\frac{1}{12}=\frac{1}{4}-\frac{1}{12}=\frac{3}{12}-\frac{1}{12}=\frac{2}{12}=\frac{1}{6}$；$\frac{2}{3}-\frac{1}{4}-\frac{1}{4}=\frac{2}{3}-(\frac{1}{4}+\frac{1}{4})=\frac{2}{3}-\frac{1}{2}=\frac{4}{6}-\frac{3}{6}=\frac{1}{6}$；$\frac{5}{8}-\frac{3}{8}-\frac{1}{12}=\frac{2}{8}-\frac{1}{12}=\frac{1}{4}-\frac{1}{12}=\frac{3}{12}-\frac{1}{12}=\frac{2}{12}=\frac{1}{6}$；$\frac{2}{3}-(\frac{1}{4}+\frac{1}{4})=\frac{2}{3}-\frac{1}{2}=\frac{4}{6}-\frac{3}{6}=\frac{1}{6}$
解析：$\frac{5}{8}-(\frac{3}{8}+\frac{1}{12})=\frac{5}{8}-\frac{3}{8}-\frac{1}{12}=\frac{2}{8}-\frac{1}{12}=\frac{1}{4}-\frac{1}{12}=\frac{3}{12}-\frac{1}{12}=\frac{2}{12}=\frac{1}{6}$；$\frac{2}{3}-\frac{1}{4}-\frac{1}{4}=\frac{2}{3}-(\frac{1}{4}+\frac{1}{4})=\frac{2}{3}-\frac{1}{2}=\frac{4}{6}-\frac{3}{6}=\frac{1}{6}$；$\frac{5}{8}-\frac{3}{8}-\frac{1}{12}=\frac{2}{8}-\frac{1}{12}=\frac{1}{4}-\frac{1}{12}=\frac{3}{12}-\frac{1}{12}=\frac{2}{12}=\frac{1}{6}$；$\frac{2}{3}-(\frac{1}{4}+\frac{1}{4})=\frac{2}{3}-\frac{1}{2}=\frac{4}{6}-\frac{3}{6}=\frac{1}{6}$
16. 怎样算简便就怎样算。
$\frac{3}{10}+\frac{3}{4}+\frac{7}{10}$ $\frac{5}{6}+\frac{2}{5}+\frac{1}{6}+\frac{3}{5}$ $\frac{8}{9}-(\frac{4}{9}+\frac{1}{3})$
$\frac{3}{10}+\frac{3}{4}+\frac{7}{10}$ $\frac{5}{6}+\frac{2}{5}+\frac{1}{6}+\frac{3}{5}$ $\frac{8}{9}-(\frac{4}{9}+\frac{1}{3})$
答案：$\frac{7}{4}$，2，$\frac{1}{9}$
解析：$\frac{3}{10}+\frac{3}{4}+\frac{7}{10}=(\frac{3}{10}+\frac{7}{10})+\frac{3}{4}=1+\frac{3}{4}=\frac{7}{4}$；$\frac{5}{6}+\frac{2}{5}+\frac{1}{6}+\frac{3}{5}=(\frac{5}{6}+\frac{1}{6})+(\frac{2}{5}+\frac{3}{5})=1+1=2$；$\frac{8}{9}-(\frac{4}{9}+\frac{1}{3})=\frac{8}{9}-\frac{4}{9}-\frac{3}{9}=\frac{1}{9}$
解析：$\frac{3}{10}+\frac{3}{4}+\frac{7}{10}=(\frac{3}{10}+\frac{7}{10})+\frac{3}{4}=1+\frac{3}{4}=\frac{7}{4}$；$\frac{5}{6}+\frac{2}{5}+\frac{1}{6}+\frac{3}{5}=(\frac{5}{6}+\frac{1}{6})+(\frac{2}{5}+\frac{3}{5})=1+1=2$；$\frac{8}{9}-(\frac{4}{9}+\frac{1}{3})=\frac{8}{9}-\frac{4}{9}-\frac{3}{9}=\frac{1}{9}$
17. 小华调查了全班同学在三八妇女节送给妈妈的礼物：$\frac{1}{4}$的同学送鲜花，$\frac{1}{3}$的同学送贺卡，其余同学送的是自己画的画。送画的同学占全班同学的几分之几？
答案：$\frac{5}{12}$
解析：$1-\frac{1}{4}-\frac{1}{3}=\frac{12}{12}-\frac{3}{12}-\frac{4}{12}=\frac{5}{12}$
解析：$1-\frac{1}{4}-\frac{1}{3}=\frac{12}{12}-\frac{3}{12}-\frac{4}{12}=\frac{5}{12}$
18. 一节课有$\frac{2}{3}$小时。同学们做实验大约用了全部时间的$\frac{1}{3}$，老师讲解大约用了全部时间的$\frac{1}{5}$，其余时间用来做作业。做作业的时间大约是整节课的几分之几？
答案：$\frac{7}{15}$
解析：$1-\frac{1}{3}-\frac{1}{5}=\frac{15}{15}-\frac{5}{15}-\frac{3}{15}=\frac{7}{15}$
解析：$1-\frac{1}{3}-\frac{1}{5}=\frac{15}{15}-\frac{5}{15}-\frac{3}{15}=\frac{7}{15}$
一根蜡烛第一次烧掉全长的$\frac{1}{5}$，第二次烧掉剩下的一半。这根蜡烛还剩下全长的几分之几？
答案：$\frac{2}{5}$
解析：第一次烧掉后剩下$1-\frac{1}{5}=\frac{4}{5}$，第二次烧掉$\frac{4}{5}×\frac{1}{2}=\frac{2}{5}$，还剩下$\frac{4}{5}-\frac{2}{5}=\frac{2}{5}$
解析：第一次烧掉后剩下$1-\frac{1}{5}=\frac{4}{5}$，第二次烧掉$\frac{4}{5}×\frac{1}{2}=\frac{2}{5}$，还剩下$\frac{4}{5}-\frac{2}{5}=\frac{2}{5}$