how long together

Lorraine's productivity is (1 project/6 hours) or 0.1667 projects per hour

Rocio's productivity is (1 project/10 hours) or 0.1 projects s per hour

If t is the time (in hours) it would take them to complete one project while working together, then

(0.1667 projects/hr)*t + (0.10 projects/hr)*t = 1

t*(0.1667 + 0.1) = 1

t*(0.2667) = 1

Therefore

**t = 3.75 hours**