WAEC 2011 · Paper 2 · Q14

The number of cars that called at a petrol station on some days of a month is as shown in the table.

Days of the month ( $x$ )	3	5	8	12	15	19	22	26
Number of cars ( $y$ )	143	95	112	110	104	86	78	69

(a)
Represent this information on a scatter diagram.
Model answer
Plot the eight points (day across, number of cars up); don't join them. The means are $\bar x = 13.75$ and $\bar y = 99.625$ : mark $(13.75, 99.625)$ and draw one straight line through it that follows the downward trend, with about as many points above it as below.

For (c), read from the line: 80 cars at about day 22, and about 73 cars on day 25. Readings vary a little with the line you draw.
(b)
Draw the line of best fit to pass through the point $(\bar x, \bar y)$ where $\bar x$ is the mean of $x$ and $\bar y$ is the mean of $y$ .
Separate values with commas, e.g. 3, −2
(c)
Use your diagram to estimate: (i) the day 80 cars called at the station; (ii) how many cars a petrol attendant at the station should expect on the 25th day.
Separate values with commas, e.g. 3, −2

The scatter diagram; draw a line through the mean point.

Worked solution (try it first)

$\bar x = \dfrac{110}{8} = 13.75$ and $\bar y = \dfrac{797}{8} = 99.625$ .
Draw the line of best fit through $(13.75, 99.6)$ , following the downward trend of the points.