[填空题]
A boat, whose speed in stil-j 40hsbt.n uml water ixrn;ou sk5x,:s 1.70 $\mathrm{~m} / \mathrm{s} $, must cross a $260-\mathrm{m}$ -wide river and arrive at a point 110 m upstream from where it starts (Fig. below). To do so, the pilot must head the boat at a 45$^{\circ}$ upstream angle.
