You place a cup of 210 degrees F coffee on a table in a room that is 68 degrees F, and 10 minutes later, it is 200 degrees F. Approximately how long will it be before the coffee is 180 degrees F? Use Newton's law of cooling:

The answer is 35 minutes

The Newton's law of cooling is:

T (x) = Ta + (To - Ta) e⁻ⁿˣ

T (x) - the temperature of the coffee at time x

Ta - the ambient temperature

To - the initial temperature

n - constant

step 1. Calculate constant k:

We have:

T (x) = 200°F

x = 10 min

Ta = 68°F

To = 210°F

n = ?

T (x) = Ta + (To - Ta) e⁻ⁿˣ

200 = 68 + (210 - 68) e⁻ⁿ*¹⁰

200 = 68 + 142 * e⁻¹⁰ⁿ

200 - 68 = 142 * e⁻¹⁰ⁿ

132 = 142 * e⁻¹⁰ⁿ

e⁻¹⁰ⁿ = 132/142

e⁻¹⁰ⁿ = 0.93

Logarithm both sides with natural logarithm:

ln (e⁻¹⁰ⁿ) = ln (0.93)

-10n * ln (e) = - 0.07

-10n * 1 = - 0.07

-10n = - 0.07

n = - 0.07 / - 10

n = 0.007

Step 2. Calculate time x when T (x) = 180°F:

We have:

T (x) = 180°F

x = ?

Ta = 68°F

To = 210°F

n = 0.007

T (x) = Ta + (To - Ta) e⁻ⁿˣ

180 = 68 + (210 - 68) e⁻⁰.⁰⁰⁷*ˣ

180 - 68 = 142 * e⁻⁰.⁰⁰⁷*ˣ

112 = 142 * e⁻⁰.⁰⁰⁷⁾ *ˣ

e⁻⁰.⁰⁰⁷*ˣ = 112/142

e⁻⁰.⁰⁰⁷*ˣ = 0.79

Logarithm both sides with natural logarithm:

ln (e⁻⁰.⁰⁰⁷*ˣ) = ln (0.79)

-0.007x * ln (e) = - 0.24

-0.007x * 1 = - 0.24

-0.007x = - 0.24

x = - 0.24 / - 0.007

x ≈ 35