Answer
88 dB.
Work Step by Step
Change the two sound level values to intensities.
$$81 dB=10log\frac{I_{81}}{I_o}$$
$$ I_{81}=10^{8.1} I_o=1.259\times10^8I_o$$
$$87 dB=10log\frac{I_{87}}{I_o}$$
$$ I_{81}=10^{8.7} I_o=5.012\times10^8I_o$$
Now add the intensities, and find the sound level.
$$I_{total}=(1.259\times10^8+5.012\times10^8)I_o=(6.271\times10^8)I_o$$
$$\beta_{total}=10log\frac{(6.271\times10^8)I_o}{I_o}=10(8.797)\approx 88dB$$
