Maria and John's Collaborative Project Time

Maria and John are collaborating on a group project. They both complete a different task that is part of the project. Maria can finish her task in 4 hours, while John can finish his task in 6 hours. Working together, they can complete both tasks in the same amount of time. Let $m$ be the rate at which Maria works (in tasks per hour), and $j$ be the rate at which John works (in tasks per hour). Set up a system of linear equations based on their work rates, and determine the total time (in hours) it takes for them to complete both tasks when working together.