Grade 8 Scatter Plots: Unit Review

Worked examples

Real problems from the practice worksheets, solved one step at a time.

1. Example 1

   x = Price ($) · y = Shirts sold

   Data: Price ($) (x) and Shirts sold (y)

   x	10	12	14	16
   y	69	69	63	55
   x	18	20	22	24
   y	51	51	49	41

   The line of best fit passes through (12, 66) and (22, 46). What is the slope of the line?

   1. Slope = change in y ÷ change in x (rise over run), using the two points on the line.
   2. Change in y: 46 − 66 = −20. Change in x: 22 − 12 = 10.
   3. Slope = −20 ÷ 10 = −2.
   4. So the slope is −2.

   Answer: −2

2. Example 2

   x = Temperature (°F) · y = Cones sold

   Data: Temperature (°F) (x) and Cones sold (y)

   x	60	64	68	72
   y	34	39	50	69
   x	76	80	84	88
   y	81	86	99	118

   How many points show a number of cones sold greater than 92?

   1. List the y-value of each of the 8 points: 34, 39, 50, 69, 81, 86, 99, 118.
   2. Greater than 92: 99, 118 (not 34, 39, 50, 69, 81, 86).
   3. Count them: 2 points.

   Answer: 2

3. Example 3

   x = x · y = y

   Data: x (x) and y (y)

   x	1	2	3	4
   y	15	21	15	13
   x	5	6	7	8
   y	15	21	31	29

   What type of association does the scatter plot show between the x-values and the y-values?

   1. Read the points from left to right (x from 1 to 8). Their y-values are 15, 21, 15, 13, 15, 21, 31, 29.
   2. Overall they go up: from 15 on the left to 29 on the right. As x increases, y tends to increase.
   3. When both go up together, the association is positive.

   Answer: positive