HalfLife when quantity is not given

[postman]

Homework Statement

"Tritium is the basic fuel of hydrogen bombs and is used to increase the power for fission bombs . . . In a basic atom bomb (or reactor), plutonium atoms are split, or fissioned, to release energy, but the fission can be promoted with a small amount of tritium because it has two extra atom-splitting neutrons. Plutonium is a relatively stable material, and its natural decay is not a major factor in bomb maintenance. Tritium, however, decays at a rate of 5.5 percent a year."
What is half life given the above?

Homework Equations

e^-rt

The Attempt at a Solution

 

[mgb_phys]

So you know that the amount of H3 reduces from 1 to 0.95 in a year.
You need a relationship between this decay rate and halflife

See http://en.wikipedia.org/wiki/Exponential_decay