If HALE = NPGK, then how will EAH be coded?

Each letter is replaced using a fixed substitution defined by the given equation HALE = NPGK:

H → N, A → P, L → G, E → K.

To code EAH, apply the same mapping letter by letter: E → K, A → P, H → N.

So EAH is coded as KPN.

(The mapping is consistent because the same letters recur in both words: E always becomes K, A always becomes P, and H always becomes N.)