Answer:
1/8 of the lawn is left to mow. As a decimal this would be 0.125.
Explanation:
Convert 1/4 to eighths, by multiplying the whole fraction by 2. The fraction would thus be 2/8. Add 5/8 and 2/8 and you know that he has mowed 7/8 of the yard, which means he now has 1/8 left to mow.