Given that the signal is 200 mW, you will convert this into Wats;
200/1000 = 0.2W
Noise = 10×2×10⁻⁶=2×10⁻⁵
Therefore the SNR = Average signal power÷Average noise power
0.2/2×10⁻⁵=10000
Thus:
SNR db = 10 ㏒₁₀ (SNR)
So in this case 10 ㏒₁₀ (10000) = 40.