Answer:
In 3 hour 45 minutes Both working together do all of yard work.
Step-by-step explanation:
Time taken by Steve to do yard work = 6 hours
Work Done by Steve in a hour = 
Time taken by Bill to do yard work = 10 hours
Work Done by Steve in a hour = 
Work done by both in a hour = 
Time taken by by both working together = 
Therefore, In 3 hour 45 minutes Both working together do all of yard work.