(x1-20)/10=-5/10=-0.5; (x2-20)/10=2.
So we have z1=-0.5 and z2=2 (these are the number of standard deviations from the mean).
Assuming a normal distribution, p(-0.5)=0.3085; p(2)=0.9772, so p(15<X<40)=0.6687 or 66.87%. This is the probability of data lying between 15 and 40.