Posted by Brandi on .
An artifact was found and tested for its carbon14 content. If 77% of the original carbon14 was still present, what is its probable age (to the nearest 100 years)? Use that carbon14 has a halflife of 5,730 years.

Algebra 
Reiny,
Since this is a halflife type, we can use the base of 1/2
.77 = (1/2)^(t/5730)
ln .77 = ln (.5^(t/5730))
t/5730 = ln.77/ln .5
t = 5730(ln.77/ln.5) = 2160.6
to the nearest 100 years that would be 2200 years. 
Algebra 
Anonymous,
The atmospheric pressure P in pounds per square inch (psi) is given by the formula below, where a is the altitude above sea level (in miles). If a city has an atmospheric pressure of 13.25 psi, what is its altitude? (Recall that 1 mi = 5,280 ft. Round your answer to the nearest foot.)