Answer
90 m.
Work Step by Step
Consider the diagram given in the book.
Let L be the distance of 65 strides that he walks upstream, (65strides)(0.8m/stride) = 52m.
Let D be the desired distance across the river. A right triangle is formed.
$$tan 30^{\circ}=\frac{L}{D}$$
$$D=\frac{L}{ tan 30^{\circ}}=\frac{52m}{ tan 30^{\circ}}\approx 90m$$
