A hypothetical A$-$B alloy of composition $58$ wt $\%$ B$-42$ wt $\%$ A at some

temperature is found to consist of mass fractions of $0\mathrm{.}5$ for both $$ and

$$ phases. If the composition of the $$ phase is $85wt\%B-15wt\%A,$ what

is the composition of the $$ phase?

$wt\%B$