Can you please explain the test case?

is one painter can only paint one board?

@sharad1103
No. A painter can paint as many boards as possible. However one board must be painted by one painter only i.e. a board cannot be divided so as to be painted half and half by two painters. Consider a testcase.
3 - No of boards
2 - No of painters
5 - Time taken to paint each board
1 3 10 - Board sizes.
The answer to this problem would be 50 only as Painter 1 can paint Board 1 in 5 units of time and Board 2 in 15 units of time , a total of 20 units of time. While Painter 2 would take 50 units of time to paint Board 3. Since the job is done in 50 units of time , the final output would be 50.
Hope this clears your doubt. Let me know if you need more help.