If 5 workers can pave 10 driveways in 30 days, how many days would it take 20 workers to pave 38 driveways?

Given that 5 workers can pave 10 driveways in 30 days, then:
let x be the number of days that can take 20 workers to complete 38 drive ways;
therefore:
proportion of work completed by 5 workers will be:
5*30/10
=15
proportion of work completed by 20 workers will be:
20*x/38
The time taken for 20 workers to complete 38 driveways will be:
15=20/38x
x=15*38/20
x=28 1/2
The answer is 28.5 days

Where is the problem