Answer: 6 times.
Explanation:
Given: Ivan earns $8 every time walks his neighbor's dog.
To earn $81, the number of times he need to walk

Therefore, the number of times he need to walk=11
If he already walked the dog 5 times.
Then he need to walk the dog =11-5=6 times
Hence, Ivan need to walk the dog 6 more times to earn $88.