$\Sigma M_O = 0$
$10N = WL$
$N = 0.10WL$
Note:
the sum of Δf will pass through point O, thus, f has no moment effect at O.
Amount of friction force
$f = \mu N = 0.20(0.10WL)$
$f = 0.02WL$
$\Sigma F_V = 0$
$f = W$
$0.02WL = W$
$L = 50 \, \text{ mm}$ answer