Mrs. Manny needs 5.2 hours to grade her students' assignments. It would take Lexi 6.4 hours to grade the assignments alone.
In one hour, Mrs. Manny completes 1 / 5.2 = 0.1923 of the work.
In one hour, Lexi completes 1 / 6.4 = 0.15625 of the work
Together, they complete 0.1923 + 0.15625 = 0.34856 of the work.
The full work would take them 1 / 0.34856 = 2.87 hours.
It would take 2.87 hours for them to finish the task together.