If Kyle can get 5/8 of a shed built in x amount of time, he can get 62.5% done; if Jack can get 1/5 of the shed built in the same time, he can 20% of it done. As a ratio:
Kyle gets the whole thing done, 100% of the way, leaving Jack lagging behind at a somewhat lower percentage of the work being done.
and cross multiply to solve for the percentage of the shed built by Jack by the time Kyle gets it completed: