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Joseph and his family were going to the beach. Joseph's sister was running late so Joseph left their house with his mom. Joseph's dad waited for his sister to get ready and left one hour later driving 75 km/hour in an effort to catch up to Joseph and mom. After
driving for four hours Joseph's dad finally caught up. How fast was Joseph's mom driving?

Select the correct response:
100 km/hour
30 km/hour
60 km/hour
50 km/hour