Answer: See explanation
Explanation:
To solve this, we have to calculate the lowest common multiple of 2 and 5. This will be:
Multiples of 2 = 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 ....
Multiples of 5 = 5, 10, 15, 20 ....
This means that he will swim and bike every 10th day together. This will be on the 10th, and 20th in October.