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)

In this problem it decreased 20%

No he does not because he can only cover 12 square feet from 15 square feet which he will still need 3 more. this is because if 3×5=15 square feet and 15-12=3 which thats the answer