A - B = B

B * C = A

D : B = E

C * C = E

C + E = A

For which numbers stand A, B, C, D and E?

A - B = B so it holds that A = B + B which implies A = 2 * B.
B * C = A and because A = 2 * B it follows that C = 2.
C x C = E so 2 x 2 = E and E = 4.
C + E = A so 2 + 4 = A and A = 6.
A = 2 * B which implies that 6 = 2 * B. From this it follows that B = 3.
D : B = E so D : 3 = 4, which implies that D = 3 x 4 = 12.

So finally one finds A = 6, B = 3, C = 2, D = 12 and E = 4.