B1. After breakfast, the sisters Anna and Birgit go to school, each to a different school. Their house is located on a bike path that runs between both schools. Anna cycles at a constant speed of $12 \mathrm{~km}$ per hour and Birgit walks in the opposite direction at a constant speed of $4 \mathrm{~km}$ per hour. They leave at the same time. Shortly after, their mother notices that the girls have forgotten their lunch and decides to catch up with them. Exactly 10 minutes after Anna and Birgit have left, mother sets off on her electric bike. She catches up with Anna first (and hands Anna her lunch) and then immediately turns around towards Birgit. When she catches up with Birgit, she gives Birgit her lunch and immediately turns around again to head back home. Mother always rides at a constant speed of $24 \mathrm{~km}$ per hour.

How many minutes after Anna and Birgit's departure is mother back home?
1. 42 Mother leaves the house 10 minutes after Anna and Birgit depart. After 10 minutes of driving, she catches up with Anna. Indeed, Anna has then cycled for 10+10 minutes, but she goes half as fast as mother. After 10 minutes of driving back, mother passes the house on her way towards Birgit. After another 6 minutes of driving, she catches up with Birgit. Indeed, Birgit has then already walked for 10+10+10+6=36 minutes, but she also goes six times slower than mother. After another 6 minutes of driving, mother is back home. That is exactly 10+10+10+6+6=42 minutes after the departure of Anna and Birgit.