decagon = 10 sides or 10 angles
Total angle is equal to 360°
360° ÷ 10 = 36°
I haven't seen the image of A, that is why, I started with angle A as 36°
∠A = 36°
∠B = 36° + 36° = 72°
∠C = 72° + 36° = 108
∠D = 108° + 36° = 144°
∠E = 144° + 36° = 180°
∠F = 180° + 36° = 216°
∠G = 216° + 36° = 252°
∠H = 252° + 36° = 288°
∠I = 288° + 36° = 324°
∠J = 324° + 36° = 360°