Answer
$16.67N$
Work Step by Step
We know that when Block B starts moving, then
$P-\mu g=0$
We plug in the known values to obtain:
$P-0.3(10)(9.81)=0$
$\implies P=294N$
When Block B starts rotating about P, then
$\Sigma M_P=0$
$\implies 10\times 9.81\times 0.15-P\times 0.40=0$
$\implies P=36.8N$
Similarly, when A starts moving, then
$P-(0.1)(17\times 9.81)=0$
$\implies P=16.67N$
This is the required force which is applied to the cord without causing motion.
