Answer:
In 15 days
Explanation:
Eva Volunteers Every 3 Days:
So, Eva volunteers on:
3rd, 6th, 9th, 12th, 15th, 18th, 21st...and so on DAYS
Jin Volunteers Every 5 Days:
So, Jin volunteers on:
5th, 10th, 15th, 20th, 25th...and so on DAYS
So, we see NEXT which day they vounteer together. Looking at the list, we see that it is on 15th day. This is also the LCM (Least Common Multiple) of 3 and 5.
So, in 15 days, they volunteer on the same day again.