Complete the words

Question

Actually these puzzles have been made by me two years ago, but I've made it easier and faster to finish by reducing the calculation operation.

        A,B,C,D,E,F G,H,I J,K,L,M,N,O,P

  3,15,14(E),20,18,15,12,12,5,4  2,25 20,8(A),5

            19,9,20,21,1,20,9,15,14

       5,14,22,9,18,15,14,13(C),5,14,20

                 1,14,4

         9,14,6,12,21,5,14,3,5

B - C = A
C + D = E
F - G = H
I + J = K
K = B
E - C = G
K - G = O
J = L = M
O - G = P
C - A = I

Find what number represents to A, B, C, etc.

Answer 1

The numbers are

The usual mapping between numbers and letters (1 = A, 2 = B, ...), yielding:
CONTROLLED BY THE
SITUATION
ENVIRONMENT
AND INFLUENCE

Taking the letters in parentheses to be

the values of the neighboring letters in the system of equations, we get:
A = 8, B = 21, C = 13, D = 1, E = 14, G = 1, I = 5, J = 16, K = 21, L = 16, M = 16, O = 20, P = 19, with F = H + 1 and N not determined.

Translating the first line in the same way as before, we get

HUMAN? A?E PUPP?TS

Filling in the remaining gaps and putting everything together, we get

HUMANS ARE PUPPETS CONTROLLED BY THE SITUATION ENVIRONMENT AND INFLUENCE
(which tells us F = 19, H = 18, and N = 5).