How do I set-up/solve this problem? Please help me.

Question

The half-life of the radioactive isotope gallium-70 is 20 minutes. If the initial amount of gallium-70 is 25 milligrams, when will 10 milligrams be left?

Answer 1

here is your same question answered.

http://www.jiskha.com/display.cgi?id=1297973911