Consider triangle WBG, where points W, B and G represent Weather Station, Balloon and Ground, respectively. This triangle is right with right angle at vertex G. The legs of this triangle are GB and GW, the hypotenuse is BW.
From the question you know that GB=4 km and GW=1 km. By the Pythagorean theorem,
[tex]BW^2=GB^2+GW^2,\\ \\BW^2=4^2+1^2,\\ \\BW^2=17,\\ \\BW=\sqrt{17}\approx 4.123\text{ km.}[/tex]
Answer: nearly 4.123 km
