Sound level question. Where am I going wrong

A firecracker produces 20mW of sound power.

Find the sound level of 3 firecrackers, 6.0m away.

So I calculated the intensity of 3 firecrackers.

I=3*P/(4pir^2)
=(3* 20e-3) / (4*pi*6.0^2)
=1.326e-4

Then, I find the sound level.

B=10log(I/Io)
=10*log(1.326e-4/10e-12)
=71.2

The answer is 81.2dB


Thanks to anyone who answers:)

Io is not 10e-12 but 1.0 e-12 . Hence you are 10 dB too low.