Question

Can someone explain to me how to solve these ?

Answer 1

$\bf 21)\\\\ \cfrac{c}{a}=r+d\implies \cfrac{c}{r+d}=a \\\\\\ 23)\\\\ ka=v+w\implies a=\cfrac{v+w}{k} \\\\\\ 25)\\\\ z=y+mx\implies z-y=mx\implies \cfrac{z-y}{m}=x$