The clot timer

A simple model says that the share of fibrin formed after t minutes is $100(1 - e^{-kt})$ per cent, where $k = 0.0008 \times$ platelets $\times$ calcium. Platelets are in thousands per mm³, and calcium is the share of the normal level. You will write the model and use it.

The idea. A larger $k$ means a faster clot.

Your tasks.

1. Finish fibrin. It gives the per cent of fibrin formed after t minutes.
2. Finish time_to_90. It gives the minutes to reach $90\%$.
3. Set least to the smallest count in the list for which, with normal calcium, the clot reaches $90\%$ within $15$ minutes.