A can build a wall in the same time in which B and C together can do it. If A and B together could do it in 10 days and C alone in 15 days, in what time could B alone do it?

Let daily work rates be $1/A$, $1/B$, and $1/C$. Given $1/A=1/B+1/C$ and $1/C=1/15$. Also, $1/A+1/B=1/10$. Substitute $1/A=1/B+1/15$ into the second equation: $1/B+1/15+1/B=1/10$. So $2/B=1/10-1/15=1/30$, hence $1/B=1/60$. Therefore B alone takes 60 days.