Mathematics - a circular lagoon is 98 meter in diameter. If you jog around it 5 times, how many?

The circumference C of a circle is

C = 2 * PI * r

r = radius of the circle
PI = 3.1415...

and the lagoon has a diameter d = 98m which is twice the radius already. Thus

PI * 98m = 307.876...m circumference of that circular pond.

5 times that distance is then 1539.38...m
or, roughly
1.539 km.