We have the next information
100 mg of menthol
every minute the amount of menthol in your body is cut in half
we have the next variables
x= minutes
y= amount of menthol in mg remaining in your body
so the equation that can we model is
then we have that It is safe to take a new cough drop after the level of menthol in your body is less than 5 mg
y= 5mg
in order to know the time we need to solve the equation above
then we isolate the x
after 5 minutes you can take another cough drop