First we have to put the information in standard units.

(0.3 - 0.502) / 0.25 = -0.808

Required area = (area between z = -0.808 and z = 0) + (area between z = 0
and z = ∞)

= (area between z = 0 and z = 0.808) + (area between z = 0 and z = ∞)

= 0.2910 + 0.5 = 0.7910

Therefore, 79.10 % is the probability of mining or with a grade greater
than 0.3%.