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.