Bonjour,
1) U(ED) = U(AB) - U(DC) = 4,5 - 2,0 = 2,5 V
2) R = U(ED)/I(EC) = 2,5/0,025 = 100 Ω
3) I(AB) = I(EC) + I(GF) = 25 + 50 = 75 mA