Final answer:
The value of λ for the exponential distribution representing the length of time the Td vaccine provides protection every 10 years is 0.0667 years^-1.
Step-by-step explanation:
The exponential distribution is usually defined as f(x|λ) = λ * exp(-λx) for x >= 0, where λ is the rate parameter.
In this case, we are given that the mean of the exponential distribution is 15 years.
The mean of an exponential distribution is equal to 1/λ.
So, 1/λ = 15 years.
Now, solve for λ:
λ = 1/15 = 0.0667 years^-1