science
posted by Angie .
how old is a mammoth's tusk if there is only 25 percent C14 remaining in the sample?
a.5700 years
b.11400 years
c.17100 years
d.22800 years

Half disappeared in one halflife.
Half that remaining disappeared in the next halflife. 
..what does that mean?

Knowing the halflife of C14 would allow you to take twice the halflife.
From searching the Internet under "C14 half life", I found that the halflife of Carbon 14 is 5730 years.
This should help a little more. Thanks for asking. 
b.11400 years

B. 11400 years

5700