Reading a line graph: totals and decreases

The line graph shows cups of coffee sold from Monday to Friday. How many cups did the cafe sell altogether over the five days? Try it first. Pause if you like. The labelled gridlines go up in hundreds. What is a dashed line halfway between two labelled lines worth? Half of one hundred is 50. Each dashed line is 50 more than the labelled line below it. Find Monday. Trace straight down to the day and across to the number axis. Monday is halfway between one hundred and two hundred, so one hundred and fifty cups were sold. Trace Tuesday to the axes. Its point is halfway between two hundred and three hundred. Tuesday is two hundred and fifty cups. Read Wednesday against the labelled line. Wednesday is two hundred cups. Thursday sits on the dashed line between three hundred and four hundred. Thursday is three hundred and fifty cups. Finally, trace Friday across to its labelled gridline. Friday is three hundred cups. We have now read all five points. Add one hundred and fifty, two hundred and fifty, two hundred, three hundred and fifty, and three hundred. The total is one thousand, two hundred and fifty cups. Work out the scale, read each point, then add the five readings.

The line graph shows cars in a car park at each hour from 8 am to 1 pm. Between which two times did the number decrease the most? Try it first. Pause if you like. The question asks for the greatest decrease. We must compare the parts that go down, from one hour to the next. At 8 am, there were one hundred cars. At 9 am, the point is halfway between two hundred and three hundred: two hundred and fifty cars. At 10 am, there were three hundred and fifty cars, halfway between three hundred and four hundred. At 11 am, there were three hundred cars. At 12 noon, there were one hundred and fifty cars. At 1 pm, there were one hundred cars. Follow the line from 10 am to 11 am. It goes down. 10 am to 11 am equals three hundred and fifty minus three hundred, which is 50. Next, follow the falling line from 11 am to 12 noon. 11 am to 12 noon equals three hundred minus one hundred and fifty, which is one hundred and fifty. Then follow the line from 12 noon to 1 pm. 12 noon to 1 pm equals one hundred and fifty minus one hundred, which is 50. Compare only the decreases: 50, one hundred and fifty, and 50. The greatest decrease is one hundred and fifty, from 11 am to 12 noon. This is the steepest line that shows a decrease. The highest point tells us the most cars at one time. A decrease compares two readings, in the direction time moves.