r = 0.25 m

N = 20 rpm = 20 * 2 * pi /60 = 2.09 rad/s

Centripetal acceleration is given by:

$$a_c = r\omega^2$$

$$a_c = 0.25 \times (2.09)^2$$

$$a_c = 1.10 \ m/s^2$$

Therefore, the centripetal acceleration of a point on the outer tip blade is 1.10 m/s2.