[填空题]
In the design of a snzf:xx 4noh8s*vra(-upe y5zjslsm(7/c+)zauiy:jr8 jrmarket, there are to be several ramps connecting different parts of the sjm:zzs7(y/jua lsrji y+ )c5 8tore.
Customers will have to push grocery carts up the ramps and it is obviously desirable that this not be too difficult. The engineer has done a survey and found that almost no one complains if the force directed up the ramp is no more than 20 N . Ignoring friction, at what maximum angle $\theta$ should the ramps be built, assuming a full $30-\mathrm{kg}$ grocery cart? $^{\circ} $
