Answer:
The faster computer can do the job in 20 mins on it own.
Step-by-step explanation:
Given:
Time taken by slower computer to do job on its own =30 minutes.
Time taken by both the computers to do the job = 12 mins.
We need to find the Time taken by faster computer to do job on its own.
Solution:
Let the the Time taken by faster computer to do job on its own be 'x'.
Now we know that;
Rate to complete the job is equal to number of jobs divided by time taken to complete the job.
Rate of faster computer = 
Rate of slower computer = 
Rate of both the computers = 
Now we can say that;
Rate of both the computers is equal to sum of Rate of faster computer and Rate of slower computer.
framing in equation form we get;

Now we will take the LCM to make the denominator common we get;

Now denominator are same so we will solve the numerator.

Hence The faster computer can do the job in 20 mins on it own.