答案：$\frac{2}{3}×\frac{3}{5}+\frac{4}{5}$
$=\frac{2}{5}+\frac{4}{5}$
$=\frac{6}{5}$
$(\frac{5}{6}-\frac{3}{4})÷\frac{1}{15}$
$=(\frac{10}{12}-\frac{9}{12})×15$
$=\frac{1}{12}×15$
$=\frac{5}{4}$
$(1-\frac{5}{6}×\frac{3}{5})÷4$
$=(1-\frac{1}{2})÷4$
$=\frac{1}{2}×\frac{1}{4}$
$=\frac{1}{8}$
$6+(\frac{2}{3}÷\frac{4}{15}-\frac{1}{4})$
$=6+(\frac{5}{2}-\frac{1}{4})$
$=6+\frac{9}{4}$
$=\frac{33}{4}$

答案：$\frac{9}{10}×\frac{5}{12}+\frac{4}{7}+\frac{3}{7}$
$=\frac{3}{8}+(\frac{4}{7}+\frac{3}{7})$
$=\frac{3}{8}+1$
$=\frac{11}{8}$
$\frac{5}{8}×\frac{5}{9}+\frac{3}{8}×\frac{5}{9}$
$=(\frac{5}{8}+\frac{3}{8})×\frac{5}{9}$
$=1×\frac{5}{9}$
$=\frac{5}{9}$
$\frac{13}{10}÷7+\frac{13}{10}×\frac{6}{7}$
$=\frac{13}{10}×\frac{1}{7}+\frac{13}{10}×\frac{6}{7}$
$=\frac{13}{10}×(\frac{1}{7}+\frac{6}{7})$
$=\frac{13}{10}×1$
$=\frac{13}{10}$
$24×(\frac{5}{6}-\frac{3}{8})$
$=24×\frac{5}{6}-24×\frac{3}{8}$
$=20-9$
$=11$

答案：120×(1 - 2/5)
=120×3/5
=72（人）
答：六年级参加啦啦队的有72人。