Need some asap- how do i work this out? x

Using the Pythagorean theorem, we can calculate the distance from B to D.

60^2 + 50^2 = X^2

3600 + 2500 = x^2
6100 = x^2

X = SQRT(6100) = 78.1 m Round to 78 ( nearest meter)

If he walks a straight line from B to D, he would walk 78 meters.
 If he walks from B to C to D he would walk 60 + 50 = 110 meters.

Since 78 is less than 110, the straight line from B to D is shorter by 100-78 = 22 meters.

What mathematical statement is missing ? ? the answer is b