Помогите плиз ) номер 19,20,21 прошуууу ребят оч надо

$20)\\ c=\frac{2}{\frac{1}{b}+\frac{1}{a}}\\ c=\frac{2}{\frac{a+b}{ab}}\\ c=\frac{2ab}{a+b}\\ ac+bc=2ab\\ b(2a-c)=ac\\ b=\frac{ac}{2a-c}\\\\ 21)\\ \frac{6}{x-y}+\frac{15}{x+y}=2\\ \frac{24}{x-y}-\frac{25}{x+y}=-9\\\\ \frac{1}{x-y}=a\\ \frac{1}{x+y}=b\\\\ 6a+15b=2\\ 24a-25b=-9\\\\ 24a+60b=8\\ 24a-25b=-9\\\\ 85b=17\\ b=\frac{1}{5}\\ a=-\frac{1}{6}\\ y-x=6\\ x+y=5\\\\ 2y=11\\ y=5.5\\ x=-0.5$

Средняя оценка равна $\frac{2*5+3*26+4*17+5*2}{5+26+17+2}=3.32\\\\$
$\frac{17+2}{50}=0.38$