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# The temperature of a certain cup of coffee 10 minutes after

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Intern
Joined: 16 Apr 2010
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The temperature of a certain cup of coffee 10 minutes after [#permalink]

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28 Jul 2010, 09:11
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The temperature of a certain cup of coffee 10 minutes after it was poured was 120 degrees Fahrenheit. If the temperature F of the coffee t minutes after it was poured can be determined by the formula F = 120*2^(-at) + 60, where F is in degrees Fahrenheit and a is a constant. Then the temperature of the coffee 30 minutes after it was poured was how many degrees Fahrenheit?

A. 65
B. 75
C. 80
D. 85
E. 90
[Reveal] Spoiler: OA
Manager
Joined: 16 Apr 2010
Posts: 210
Re: The temperature of a certain cup of coffee 10 minutes after [#permalink]

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28 Jul 2010, 11:57
Hi,

Since F=120 (2^-at)+60,
replace F=120, t=10 to find a.
Once you find a, then to find the temperature 30 min after it was poured simply replace
t=30 and value of a in the equation.

regards,
Jack
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Re: The temperature of a certain cup of coffee 10 minutes after [#permalink]

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28 Jul 2010, 13:26
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Is the question correctly written?? I am unable to find the solution.

$$F= 120* (\frac{1}{2^at}) + 60$$
Is this formula correct?
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Re: The temperature of a certain cup of coffee 10 minutes after [#permalink]

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29 Jul 2010, 04:03
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kilukilam wrote:
The tempreature of a certain cup of coffee 10 minutes after it was poured was 120 degree Faranite. If the temp F of the coffee t minues after it was poured can be determinde by the formula F=120 (2^-at)+60 , where F is degree Fahrenhite and a is constant, then the tempreature of the cofffee 30 minues after it was poured was how many degree fahrenhite

a. 65
b. 75
c. 80
d. 85
e. 90

We have the formula for calculating temperature: $$F=120*2^{-at}+60$$ and the value for $$t=10$$ --> $$F(t=10)=120=120*2^{-10a}+60$$ --> $$\frac{1}{2^{10a}}=\frac{1}{2}$$ --> $$10a=1$$ --> $$a=0.1$$. So formula is $$F=120*2^{-0.1t}+60$$

Now for $$t=30$$ the temperature in Fahrenheits will be $$F=120*2^{-0.1*30}+60=\frac{120}{2^3}+60=75$$.

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Re: The temperature of a certain cup of coffee 10 minutes after [#permalink]

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12 Mar 2014, 05:11
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First, we have to find a.

We know that after t=10 minutes the temperature F=120 degrees.

Hence:
120 = 120*(2^-10a)+60
60 = 120*(2^-10a)
60/120 = 2^-10a
1/2 = 2^-10a
2^-1 = 2^-10a
-1 = -10a
1/10 = a

Now we need to find F after t=30 minutes:
F = 120*(2^-1/10*30)+60
F = 120* (2^-3) +60
F = 120* (1/2^3) +60
F = 120*1/8 +60
F = 15+60 = 75

Intern
Joined: 04 Jul 2013
Posts: 38
Schools: ISB '16
Re: The temperature of a certain cup of coffee 10 minutes after [#permalink]

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17 Apr 2014, 01:36
Substitute the values f=120 and t=10 in given equation, this gives value of a=0.1

now equation is f=120(2^-0.1t)+60

substitute t=30 in above equation to find f

Intern
Joined: 06 Mar 2015
Posts: 7
Re: The temperature of a certain cup of coffee 10 minutes after [#permalink]

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27 Mar 2015, 06:34
Can someone explain me how
1/2 = 2^-10a becomes

2^-1 = 2^-10a
Intern
Joined: 17 Feb 2015
Posts: 28
GPA: 3
Re: The temperature of a certain cup of coffee 10 minutes after [#permalink]

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27 Mar 2015, 06:59
lzielen wrote:
Can someone explain me how
1/2 = 2^-10a becomes

2^-1 = 2^-10a

Hi lzielen!

$$\frac{1}{2}$$ is $$2^{-1}$$.

$$x^{-n} = 1/x^n$$
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Re: The temperature of a certain cup of coffee 10 minutes after [#permalink]

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08 Oct 2017, 12:13
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Re: The temperature of a certain cup of coffee 10 minutes after   [#permalink] 08 Oct 2017, 12:13
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