The mean lifetime of a TV set(T, in years) is 10 years.

I need help with a few hw questions:

1. The mean lifetime of a TV set(T, in years) is 10 years. If this TV set has lasted 10 years, what is the probability that it will last for at least another 10 years?

So this looks like a normal distribution curve question to me. f(t) = ((1/sigma x Sqrt of 2 x pi)) x e^-(1/2(t-u/sigma)). I can plug in the mean, and the t to get f(t) = ((1/sigma x Sqrt of 2 x pi)) x e^-(1/2(10/sigma)). But I not having the value for sigma is screwing me up and I don't know what equation I can apply to figure it out or whether I just started with the wrong equation