If the block is moving at a constant velocity, it means there is no net force, which means the frictional force is same in magnitude as the pulling force!
F_pull = Friction
Now the frictional force (F = cR) is related to the coefficient of friction & normal force, where the mass of block is important (R = mg). Putting all these small equations together:
F_pull = cR
F_pull = cmg
Let's plug in the values we know
(30) = c (15)(9.8)
... and use the calculator to find that coefficient, c!
