Then 1 window 15 minutes 15 copies, 60 copies in 30 minutes in 2 windows, with a difference of 45 copies.

Because the queuing customers are all full in the lobby where the two processing methods end, the number of customers handled by the two windows is equivalent to 15 minutes of queuing customers.

So the number of customers waiting in line per minute is: 45÷ 15=3.

Therefore, at least three windows should be opened to ensure on-call service.

Comprehensive formula: (30× 2-15×1) ÷ (30-15) = 3.

(2) The total distance between the three villages is 8+7+5=20.

The ratio of labor force between village A and village B is 60: 40 = 3: 2.

So after the completion, the distance ratio between the two villages is also 3: 2.

Therefore, a village repair: 20×3/5= 12, and B village repair: 20×2/5=8.

Because Jia Cun has to repair 8 books for free, Jia Cun has to repair 4 more books; B village repair 1 copy.

Therefore, the two villages should pay according to the ratio of 4: 1.

Therefore, A village deserves 1350×4/5= 1080 yuan, and B village deserves 1350- 1080=270 yuan.

(3) Because the bottom of the cone is the same as the cylinder, the volume formula of the cone is multiplied by 1/3.

Therefore, a cone and a cylinder with the same volume, if the bottom surface is the same, the height of the cone must be three times that of the cylinder.

So a 6 cm high cone is only equivalent to the water that a 2 cm high cylinder can hold.

So there is still water in the tank: 7-2=5 cm deep.

So the total height of water is 5+6= 1 1 cm.