Asubstance decays 18℅ each day. after 11 days, there are 13 milligrams of the substance remaining. how many milligrams were there initially? round to two decimals

The amount remaining (Q(t)) can be expressed in terms of the initial quantity Q₀ and the number of days (t) by
  Q(t) = Q₀×0.82^t

Filling in the given numbers, you have
  13 = Q₀×0.82^11
  13/0.82^11 = Q₀
  Q₀ ≈ 115.34 . . . . milligrams

The decay formula is as follows:
M(t)=M(0)((1-r)^t)  
where
M(t)=mass of substance after t days
r=rate of decay

Substitute r=0.18, t=11, we get
M(11)=M(0)(1-0.18)^11
=>
M(0)=(13 mg) / (0.82^11) = 115.34 mg (initial mass of substance)

If you make you answer for clear i can answer it : )

The answer is that the rate of change is $50 per 1 monthhope this ; )