24. a radioactive substance decays exponentially. a scientist begins with 110 milligrams
of a radioactive substance. after 31 hours, 55 mg of the substance remains. how
many milligrams will remain after 42 hours?

Firstly set up the equation as so:
R = me^(-λt)
From the initial conditions: 
 110 = me^(-λ*0)
∴ m = 110

From the other piece of information:
55 = 110e^(-λ*31)
55/110 = e^(-31λ)
∴ -ln(2) = -31λ
λ = ln(2)/31

Therefore:
R = 110e^(42*(-ln(2)/31))
∴ R ≈ 43 mg

(ln(x) is the natural log of x)

Ais c multiple rthvjjbv