Answer:
The ambient temperature is 35°C.
It takes 15 minutes to cool the substance from 40°C to 35°C.
Step-by-step explanation:
Using Newton's Law of Cooling to answer the given problem.

Given:
The time it takes to cool from 50°C to 45°C = 3 minutes
The time it takes to cool from 45°C to 40°C = 5 minutes
Find:
Time ambient temperature and the time it takes to cool the substance from 40°C to 35°C
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(1) - Using first initial condition:

(2) - Using the second initial condition:

(3) - Now we have a system of equations.

(4) - Solve the system by dividing the top equation by the bottom equation.

Thus, the ambient temperature is 35°C.
(5) - Find the value of "k" using either of the two previous equations

(6) - Now finding "dt"

Thus, it take 15 minutes to cool the substance from 40°C to 35°C.