Gprep_ temperature!!

[nazar]

The temperature of a certain cup of coffee 10 minutes after it was poured was 120 degrees Fahrenheit. If the temperature 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?

• 65
• 75
• 80
• 85
• 90

OA: B.

[Ankost]

B.
We need to find constant a.
120*(1/2)^10a + 60=120
2*(1/2)^10a=1
2=2^10a, a=1/10
Plug a=1/10 and t=30 into the formula
120*1/8+60=75

[GmatG]

Originally Posted by Ankost

B.
We need to find constant a.
120*(1/2)^10a + 60=120
2*(1/2)^10a=1
2=2^10a, a=1/10
Plug a=1/10 and t=30 into the formula
120*1/8+60=75

Same here.. the key to all such questions are to find the constant(a in this case) first .

[ddyno]

I got the same ques in GMATPrep, though I got it wrong that time!! But later applied the same technique and got the ans=75.

[nikhilsikri]

75 ...is the ans ... first equation gives a

[NishantG]

formula

[120*(2^-at)]+60
After 10 minutes
[120/(2^10a)]+60 Since this is equal to 120 2^10a should be equal to 2 so a has to be 1/10

At t=30

120/(2^30a) + 60

Substituting value of a we get answer as 75

[btit]

Does this kind of problems try to test how quickly you can solve linear equations?

This question gives simple formula with 1 variable (a) and big nos (120, 60, 30, etc..)

The earlier we get this into single no, the quicker you reach to soln.

120 = 120/2^10a + 60
2 = 2^10a
so, 10a = 1 => a=1/10.
2nd sqn with t=10 gives F=75.

[ssky]

Calcuation are not too big in this question and I think that in GMAT you do need to do some calculations doable with a minute or so. In this example, I got the answer probably in 30 to 50 sec. Key is to know what to do like Ankost said..you have to know instantly that you have to find the constant a and then solve for the temp. My 2 cents

[raunekk]

i will go with B too...