The first day Ivan and Gavin will go to the gym at the same time is on the 15th day.
Step-by-step explanation:
To solve this, we have to find the LCM (least common multiple) of 3 and 5, because we want the numbers to land together. To do this, let's list the first couple multiples of 3 and 5
3: 3,6,9,12,15,18,21
5: 5,10,15,20,25,30
As you can see, 15 is the first common multiple we see on the list.