A computer can perform c calculations in s seconds. How many minutes

(s sec/c calculations)(1 min/60 sec)(k calculations) = (ks)/(60c) min

Answer C

Information in the question is given in seconds, answer asked in minutes.

First, convert the given information in minutes

c calculations are performed in \(\frac{s}{60}\) minutes

Time required for k calculations \(= \frac{s}{60} * \frac{k}{c} = \frac{sk}{60c}\)

Answer = C

MAGOOSH OFFICIAL SOLUTION:

Hi All,

As 'thick' as this question might look, it's actually a fairly straight-forward Rate question and can be solved by TESTing VALUES.

We're told that a computer can perform C calculations in S seconds.

Let's TEST....
C = 2
S = 3
2 calculations every 3 seconds =
40 calculations every 1 minute

We're asked how many MINUTES it will take to perform K calculations.

Since we already know that the computer can perform 40 calculations in 1 minute, let's TEST...

K = 80

So we're looking for an answer that = 2 when C = 2, S = 3 and K = 80

Answer A: 60KS/C = 60(80)(3)/2 = 7200 This is NOT a match
Answer B: KS/C = (80)(3)/2 = 120 This is NOT a match
Answer C: KS/60C = (80)(3)/(60)(2) = 2 This IS a match
Answer D: 60C/KS = 60(2)/(80)(3) = 1/2 This is NOT a match
Answer E: K/60CS = 80/(60)(2)(3) = a fraction. This is NOT a match

Final Answer:
GMAT assassins aren't born, they're made,
Rich

Work=Rate*time, c=Rate*s
Rate=c/s calc/sec or 60c/s calculations/minute
Now,after calculating the rate,
Work=Rate*Time,this time k=60c/s*time
Or time=ks/60c

Unitary method is the key.

c Calculations done in s seconds
1 calculation is done in (s/c) seconds
k calculations r done in (ks/c) seconds

Answer in minutes: (ks/60 c) minutes.

C is the answer.

In such questions, I usually like to plug in very simple values.
Say 1 ( = c) calculation in 10 (=s) secs. Then 2 (= k) calculations in 20 secs (= 1/3 mins).

Look for the option which gives 1/3 for c = 1, s = 10 and k = 2. From the numbers, it is obvious that we only need to check for (C) and (E) since 60 should be in the denominator to give us 3 in the denominator in the result (there is no other multiple of 3).

Answer (C)

w = r*t
c = r*s
r = c/s

k = r*t
k = (c/s)*60t
t = ks/60c

Hi KarishmaB, thank you for sharing this interesting approach which I think would be benificial if I can understand it.
Do you mind to elaborate further on the bolden part; especially on how come you can eliminate options other than C & E because other options denominator don't contain 60? 

Thanks before

I need to plug in values: c = 1, s = 10 and k = 2 in each option. The option that gives us 1/3 is  the answer. Now look at the options.
To plug in values 

\(A. \frac{60ks}{c}\)

I need 1/3. If c = 1, the denominator here is 1. Can I get 1/3? No. k and s are both integers. HOw will I get 3 in denominator?
So not possible. Ignore. 
Same logic for (B) too.

\(D. \frac{60c}{ks}\)

k = 2 and s = 10 so ks in denominator is 20. It doesn't have a 3 as a factor. How will I get 3 in the denominator? c is an integer. Hence ignore. 

(C) and (E) both have 60 in denominator. 60 = 20 * 3 so there is a 3 in the denominator. Now I can get 3. Check for both. ­