In a circle, there is a chord measuring 20 cm that is located 15 cm away from the center. what is the approximate radius of the circle?

The perpendicular from the center of a circle to a chord bisects the chord.
These form a right-angled triangle with the radius as the hypotenuse.
Length of chord= $ \dfrac{20}{2}=10 $
Distance of chord from the center = 15 cm
Using Pythagoras theorem:

$ r^2=15^2+10^2 $
$ r^2=225+100=325 $
$ r=\sqrt{325} = 18\ \text{cm} $