$M+F=300$
Normally, we're used to equations like $x=300$. Here, consider $M+F$ a single variable and try to manipulate the solution such that you extract $M+F$ somehow.
We need the value of
$750*\frac{8}{9}M+1000*\frac{2}{3}F$
=> $\frac{6000}{9}M+\frac{2000}{3}F$
=> $\frac{6000}{9}M+\frac{6000}{9}F$
=> $\frac{6000M + 6000F}{9}$
=> $\frac{6000(M +F)}{9}$
=> $\frac{6000(300)}{9}$
Option B