Question 6
Dad walks at $6$ km per hour when alone and mom walks at $4$ km per hour when alone. When they walk together, they compromise at $5$ km per hour. They leave home to go to the store which is $1$ km away. Six minutes after leaving home, dad has to return for the shopping list while mom goes on. How long does mom have to wait in the store, in minutes, before dad arrives with the shopping list?