81°.
The law of reflection states that the angle of incidence is equal to the angle of reflection.
When sounds at two different frequencies are combined,
two new sounds are created ... at the sum and difference
of the original frequencies.
Combining two sounds at 490 Hz and 488 Hz creates
beats at 978 Hz and 2 Hz.
The fluttering "wah wah" effect of the 2 Hz beat is much more
noticeable than the new sound at 978 Hz.
From reliable sources in the internet, the half-live of carbon-14 is given to be 5,730 years. In a span of 10,000 to 12,000 years, there are almost or little more than 2 half-lives. Thus, there should be
A(t) = A(0)(1/2)^t
where t is the number of half-lives, in this case 2. Thus, only about 1/4 of the original amount will be left.
