x - производительность Олжаса, y - Димы, z - Антона.
Олжас и Дима нарубят за 10 дней, т.е. (x+y)*10=1.
Дима и Антон за 15 дней, т.е. (y+z)*15=1.
Олжас и Антон за 18 дней, т.е. (x+z)*18=1.
Имеем 3 уравнения с 3мя неизвестными:
![\begin{cases} (x+y)\cdot10=1\\ (y+z)\cdot15=1\\ (x+z)\cdot18=1 \end{cases}\Rightarrow \begin{cases} x=\frac1{10}-y\\ z=\frac1{15}-y\\ (\frac1{10}-y+\frac1{15}-y)\cdot18=1 \end{cases}\\ (\frac1{10}-y+\frac1{15}-y)\cdot18=1\\ \left(\frac16-2y\right)\cdot18=1\\ 3-36y=1\\ 36y=2\\ y=\frac2{36}=\frac1{18}\\ \begin{cases} x=\frac1{22,5}\\ z=\frac1{90}\\ y=\frac1{18} \end{cases}\\ \begin{cases} (x+y)\cdot10=1\\ (y+z)\cdot15=1\\ (x+z)\cdot18=1 \end{cases}\Rightarrow \begin{cases} x=\frac1{10}-y\\ z=\frac1{15}-y\\ (\frac1{10}-y+\frac1{15}-y)\cdot18=1 \end{cases}\\ (\frac1{10}-y+\frac1{15}-y)\cdot18=1\\ \left(\frac16-2y\right)\cdot18=1\\ 3-36y=1\\ 36y=2\\ y=\frac2{36}=\frac1{18}\\ \begin{cases} x=\frac1{22,5}\\ z=\frac1{90}\\ y=\frac1{18} \end{cases}\\](https://tex.z-dn.net/?f=%5Cbegin%7Bcases%7D+%28x%2By%29%5Ccdot10%3D1%5C%5C+%28y%2Bz%29%5Ccdot15%3D1%5C%5C+%28x%2Bz%29%5Ccdot18%3D1+%5Cend%7Bcases%7D%5CRightarrow+%5Cbegin%7Bcases%7D+x%3D%5Cfrac1%7B10%7D-y%5C%5C+z%3D%5Cfrac1%7B15%7D-y%5C%5C+%28%5Cfrac1%7B10%7D-y%2B%5Cfrac1%7B15%7D-y%29%5Ccdot18%3D1+%5Cend%7Bcases%7D%5C%5C+%28%5Cfrac1%7B10%7D-y%2B%5Cfrac1%7B15%7D-y%29%5Ccdot18%3D1%5C%5C+%5Cleft%28%5Cfrac16-2y%5Cright%29%5Ccdot18%3D1%5C%5C+3-36y%3D1%5C%5C+36y%3D2%5C%5C+y%3D%5Cfrac2%7B36%7D%3D%5Cfrac1%7B18%7D%5C%5C+%5Cbegin%7Bcases%7D+x%3D%5Cfrac1%7B22%2C5%7D%5C%5C+z%3D%5Cfrac1%7B90%7D%5C%5C+y%3D%5Cfrac1%7B18%7D+%5Cend%7Bcases%7D%5C%5C)
Олжас нарудит дрова за 18 дней, Дима за 22,5, Антон за 90 дней.