Anna and Bob are baking cookies to sell. Anna can bake a batch of cookies in 4 hours, and Bob can bake a batch in 6 hours. They decide to work together to bake enough cookies for a community event. If they need to bake 15 batches of cookies, how long will it take them working together? Express the answer as a fraction of an hour. Let's think step by step and output the final answer within \boxed{}.
user