Solution:(Letters at odd places → Place value of the alphabet), (letters at even places → place value of the alphabet +1)
RAIN = (R = 18), {(A = 1) → (1 + 1)}, (I = 9), {(N = 14) → (14 + 1)} ⇒ 18 - 2 - 9 - 15
SUN = (S = 19), {(U = 21) → (21+ 1)}, (N = 14) ⇒ 19 - 22 - 14
Similarly,MOON = (M = 13), {(O = 15) → (15+ 1)}, (O = 15), {(N = 14) → (14 + 1)} = 13 - 16 - 15 - 15