An exponential growth/decay word problem. If spraying of grasshoppers

[HT3]

Homework Statement

If spraying of grasshoppers with 800kg/km^2 of a certain insecticide will kill 40% of these insects, how much insecticide is needed to kill 98% of the grasshoppers?
(assume exponential relationship)

Homework Equations

$$\int$$dP/dt=$$\int$$kp
lnP=kt+c
e^(kt+c)=P

The Attempt at a Solution

I set up an equation relating the amount of insecticide to how many die (exp decay -k)
800kg/km^2=Po(e^-k(.4))
im unsure as where to go from here...or if I am even on the right path

 

[rootX]

insecticide is the input not the output.
So,

G_alive/G_intial = exp(-k.insecticide used)

when insecticide used = 0 then G_alive/G_initial = 1 .. (no G killed)