Answer
$s_B=6.67m$
Work Step by Step
We can determine the required distance as follows:
We apply the conservation of linear momentum
$m_Av_A+m_Bv_B=m_Av_A^{\prime}+m_Bv_B^{\prime}$
We plug in the known values to obtain:
$2000v_A+10000v_B=2000(0)+10000(0)$
This simplifies to:
$v_A=-5v_B$
$\implies s_A=-5s_B~~~~$eq(1)
The relative distance between A and B is $40m$
$\implies S_A=s_B+s_{A/B}$
$\implies s_A=s_B+40~~~~$eq(2)
We plug in the value of $s_A$ from eq(1) into eq(2) to obtain:
$-5s_B=s_B+40$
This simplifies to:
$s_B=6.67m$
