Answer:
3.085 [m].
Explanation:
1) The rule:
m₁*g*l₁=m₂*g*l₂, where m₁ and l₁ - the mass and distance for the small child, m₂ and l₂ - for the big child;
2) according to the condigion l₁+l₂=5, then
3) it is possible to make up the system:
[tex]\left \{ {{l_1+l_2=5} \atop {m_1*l_1=m_2*l_2}} \right. \ = > \ \left \{ {{l_1=5-l_2} \atop {18*(5-l_2)=29*l_2}} \right. \ = > \ \left \{ {{l_1=\frac{145}{47}} \atop {l_2=\frac{90}{47}}} \right.[/tex]
4) finally, l₁=145/47≈3.085 [m].