11. Temperature \( F \) changes according to the differential equation
\( \frac{dF}{dt} = kF \), where \( k \) is a constant and \( t \) is measured in minutes.
If at time \( t = 0 \), \( F = 180 \), and at time \( t = 16 \), \( F = 120 \),
what is the value of \( k \)?
