A bamboo shoot doubles its height each day. If it’s 40 decimeters on the tenth day, on what day was it 5 decimeters?

Day

We don’t know the height of the bamboo shoot at its birth, so we have to work backwards, from the tenth day. If its 40 dm on the tenth day, it must have been 20 dm on the ninth day. If it was 20 dm on the ninth day, it must have been 10 dm on the eighth day. If it was 10 dm on the eighth day, it must have 5 dm on the seventh day.