Naughty and grandpa Zhang live in the same unit building, and one day they go upstairs at the same time. When Naughty walked to the third floor, Grandpa Zhang walked to the second floor, which means Naughty walked twice as fast as Grandpa Zhang (Naughty walked through two ladders when he walked to the third floor, and Grandpa Zhang walked through one ladder when he walked to the second floor).

(7-1) ÷ (3-1) ÷ (2-1) = 6 ÷ 2 = 3 (layer)