A travel mug of 86 degrees coffee is left on the roof of a parked car on a cold winter day. The temperature of the coffee after t minutes is given by H=86(2)^−t/13.

Question

After how many minutes will the coffee be only lukewarm (30 degrees Celsius)?

Answer 1

30 = 86(2^-t/13)

0.348837209 = 2^-t/13

log(0.348837209) = log(2^-t/13)

log(0.348837209) = (-t/13)log2

-t/13 = -1.519374159

t = 19.75 minutes