**Time and Work — formulas used here.** - Work done in unit time. If a worker finishes a job in $m$ days, the fraction of work done in one day is $\tfrac{1}{m}$. - Combined work (two workers). A finishes in $X$ days, B in $Y$ days; together they finish in $\tfrac{XY}{X+Y}$ days. - Combined work (three workers). A, B, C take $X$, $Y$, $Z$ days; together they take $\tfrac{XYZ}{XY+YZ+ZX}$ days. - Efficiency–time inverse. If A is $k$ times as efficient as B, A takes $\tfrac{1}{k}$ as long. - Workforce equivalence. $M_1 D_1 T_1 = M_2 D_2 T_2$ for the same work, where $M$ = workers, $D$ = days, $T$ = hours/day. **Question.** A finishes a job in 10 days and B finishes the same job in 15 days. Working together, how many days will they take?