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Joselyn is a manager of a window installing company. She ask two professional installers, Mark and Anna, in install all the windows in a large home. Mark can install all the windows in 12 hours. Anna can install all the windows in 9 hours. Joselyn wants to know how long it will take them to install all the windows in the home if they work together. Write an equation and solve for the time it will take Mark and Anna to install all the windows together. Explain each step.