Grandma Obana walked up the stairs at the same time. When grandma went to the second floor, Xiaohua had already gone to the third floor. Grandma went to the fourth floor. What floor did the joke go to

Grandma Obana walked up the stairs at the same time. When grandma went to the second floor, Xiaohua had already gone to the third floor. Grandma went to the fourth floor. What floor did the joke go to? Ask for an explanation, Pure explanation, the formula is listed by itself:

1. When Grandma went to the second floor, Xiaohua had already gone to the third floor, saying that Grandma went to the first floor and Xiaohua went to the second floor. Grandma went to the fourth floor, actually to the third floor, so Xiaohua went to the sixth floor and the seventh floor.

2. The water intake of the water inlet pipe in one hour is one sixth of that of the pool, and the water discharge of the water outlet pipe in one hour is one fifteenth of that of the pool. If it is opened at the same time, the water intake in one hour is one sixth of the pool minus one fifteenth, which is equal to one tenth of the pool. If it is filled in one hour, it will be filled in ten hours.