A is able to complete a task in 15 days, while B takes 20 days to finish the same task. If they collaborate and work together for 4 days, what fraction of the work will still remain?

1/4
7/15
8/15
11/15

উত্তর দেখান ও ব্যাখ্যা

উত্তর: (C) 8/15

ব্যাখ্যা

Together they do 1/15 + 1/20 = 7/60 per day. In 4 days that is 28/60 = 7/15, so 8/15 of the work remains. (Verified against the official SSC answer key.)