Final answer:
After 3 days of working together, 53/60th of the job will be left.
Step-by-step explanation:
To find out how much of the job will be left after 3 days of working together, we need to calculate the combined work done by A and B in 3 days and subtract it from the total work. A can complete the job in 15 days, so in 1 day, A completes 1/15th of the job. Similarly, B can complete the job in 20 days, so in 1 day, B completes 1/20th of the job.
To calculate the combined work done in 3 days, we add the individual work done by A and B: 1/15 + 1/20 = 7/60. So in 3 days, A and B together will complete 7/60th of the job.
To find out how much of the job will be left, we subtract the work done in 3 days from the total work: 1 - 7/60 = 53/60. Therefore, after 3 days, 53/60th of the job will be left.