So if it is reduced by 40% in 35 hours then it has 60% remaining after 35 hours. Using our basic formula
If our starting amount (A0) is 100 then our amount remaining (A) is 60, if our starting amount (A0) is 1 then our amount remaining (A) is 0.6. We can also just leave A0 and use A=0.6(A0). Whatever makes the most sense to you.
Using A=0.6(A0), t=35
Divide both sides by A0 (This step whatever choice you made for your starting amount will cancel out and we will get the same result.
Take the ln of both sides
divide by 35
plug this into equation
Now that we have the expression we can solve for t when A=0.5A0 (Or A=0.5 A0=1 Or A=50 A0=100)
Follow same steps as above but this time we are solving for t
Take ln of each
Multiplty by 35, divide by ln(0.6)
You could have found k and rounded to a decimal and used that instead of tracking through ln(0.6)/35 if you prefer.