[填空题]
In the design of a supermarket, thercz+984;tx nwfgj b- xxe are to be several ramps cod5ulc8 jtw19bza14 viph8a y8nnecting different parts of the s1i8dh5t89 1l a cuw azybj4pv8tore.
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} $
