How to I use the standard normal table to get the following Z value?

So I am given that P(X≤31.5)=0.05 and according to the textbook, after standardizing and using this standard normal table we get

(31.5−mean)/(standard deviation)=−1.645.

Can someone explain to me how they got -1.645 from 0.05? I udnerstand that the left side is just plugging in X into the formula, but how did we turn 0.05 into -1.645?