Mitä lukua kirjaimet vastaavat?

a.
A + B = 20
A – B = 4
A =
B =

b.
C + D = 15
C – D = 1
C =
D =

c.
E · F = 18
E : F = 2
E =
F =

d.
G : H = 5
G · H = 125
G =
H =