In this question we have a buoy attached to
an anchor. The forces act on the entire system.
The buoy has a buoyancy B_B, the anchor has
a buoyancy B_A and the anchor has a weight
W. So W = mg. B_A is equal to the weight of
water displaced by the anchor, which is the
density of water, times by the volume of the
anchor, m/rho times g. B_B = rho_w*g*V_B which
in this case is 4/3*pi*r^3. Resolving vertically
B_B + B_A = W. Substitute the values of B_B,
B_A and W into the final equation and solve
for R.
