algebra
posted by bill .
if mary takes 3 hours to mow a lawn and sally takes 5 hours how long will it take them to mow it together

Mary's rate = lawn/3
Sally's rate = lawn/5
combined rate = lawn/3 + lawn/5
= 8lawn/15
time = lawn/(8lawn/15) = 15/8 or 1.875 hrs = 1 hour and about 53 minutes