P-Value is the probability of your x given the Null hypotheses (H0) is true.

Here your H0 is Normal(20,5/sqrt(25)) (do you understand why?) = Normal(20,1)

Normalize x to obtain ((x - mean)/stdev), in this case ((21 - 20)/1) = 1

and use your tables or whatever to find the Probability of observing 1 under the Normal(0,1) distribution.

I get 0.841