Probablility of 2 means?

Two independent experiments are being run in which two different types of paints are compared. Eighteen specimens are painted using type A and the drying time in hours is recorded on each. The same is done with type B. The population standard deviations are both known to be 1.0.

Assuming that the mean drying time is equal for the two types of paint, find P(XA–XB>0.3), where XA and XB are the average drying times for samples of size nA = nB = 20. XA and XB has the same mean.



σ between 2 means = sqrt ( 2x(1/sqrt(20))^2 ) = 0.316


P(XA–XB>0.3) = P ( z > ( 0.3 - 0) / 0.316) = 0.171 , but the an sprovided is 0.00135 , is my ans wrong ?