The ascension can be modeled using the function:

Where d is the number of feet below the sea level at time t (in seconds), d₀ is the initial "depth", and r is the ascension rate.
From the problem, we identify:

Then:

a)
After 10 seconds, we have t = 10:

After 10 seconds, we will be 80 feet below sea level.
b)
To find how long will it take to reach the surface, we need to solve the equation d(t) = 0.

We will reach the surface after 30 seconds.