Half-life of tritium is 12.32 years.

Write decay equation as

N(t) = N(subzero)e^(-rt)
where t is the time in days, N(subzero) is the starting amt of tritium and N(t) is amt of tritium at time t.

don't know how to find -r.

To find the value of -r, you can use the half-life equation for radioactive decay. The half-life of a radioactive substance is the time it takes for half of the substance to decay.

In this case, the half-life of tritium is given as 12.32 years. We can use this information to find the decay constant, denoted as -r.

The half-life equation is given as:

N(t) = N₀ * e^(-rt)

Where:
N(t) is the amount of tritium at time t
N₀ is the starting amount of tritium
r is the decay constant (which we need to find)
t is the time in days

Since we know the half-life, we can use it to find the decay constant. The relationship between the decay constant and the half-life can be expressed as follows:

t(1/2) = ln(2) / r,

Where ln(2) is the natural logarithm of 2, which is approximately 0.693.

Plugging in the given half-life value of 12.32 years into the equation, we have:

12.32 = ln(2) / r

Rearranging the equation to solve for -r, we get:

r = -ln(2) / 12.32

Therefore, the value of -r for tritium is approximately -0.056, rounded to three decimal places.

Now that we have the value of -r, we can use it in the decay equation to calculate the amount of tritium at any given time t.