Let 'x' be the rate of each worker
the work done is equal to the number of workers times rate times time(duration). In this case, you are given with the number of workers, time and work done.
Let's find the rate of each worker.
since we already have the rate then we will now consider the second situation wherein there are 10 workers and they are to make 30 computers. you are asked to find the duration or the time. using the same concept, you'll have it as:
let 't' be the number of days