A tank was filled with 160 ℓ of water at 17 00. Water flowed out of the tank from 17 00 to 21 00. The line graph shows the amount of water in the tank from 17 00 to 21 00. Which one-hour period shows the least decrease in the amount of water in the tank? Give your answer in this format. (Eg 12 00 to 13 00)


To find the least decrease in a line graph, look for a decreasing slope that is closer to zero or nearly flat. This indicates that the least decrease in the amount of water in the tank.

To show:

Between 17 00 and 18 00:
Volume of water that flowed out
= 160 - 115
= 45 ℓ

Between 18 00 and 19 00:
Volume of water that flowed out
= 115 - 75
= 40 ℓ

Between 19 00 and 20 00:
Volume of water that flowed out
= 75 - 70
= 5 ℓ

Between 20 00 and 21 00:
Volume of water that flowed out
= 70 - 40
= 30 ℓ

One-hour period that shows the least decrease in the volume of water = 19 00 to 20 00 

Answer(s): 19 00 to 20 00