Substitute the initial amount 100 for 'a'. Substitute 85 for 'y' and 4 for 't'.
Solve for k.
Since we want to find how long it takes for half to decay, solve: 1 = 2e−0.0406t
This indicates an amount of 2 was reduced to 1.
y = aekt
85 = 100e4k
k = −0.0406
t = 17.06 hours