WAEC 2009 · Paper 2 · Q15

The table shows the corresponding values of two variables $x$ and $y$ .

$x$	33	31	28	25	23	22	19	17	16	14
$y$	4	6	4	10	12	10	14	15	18	22

(a)
Plot a scatter diagram to represent the data.
Model answer
Plot the ten points $(33, 4), (31, 6), (28, 4), (25, 10), (23, 12), (22, 10), (19, 14), (17, 15), (16, 18), (14, 22)$ with $x$ across and $y$ up. Don't join them. The points fall from left to right.
(b)
Calculate $\bar x$ , the mean of $x$ , and $\bar y$ , the mean of $y$ .
Separate values with commas, e.g. 3, −2
(c)
Draw the line of best fit to pass through $(\bar x, \bar y)$ .
Model answer
Plot the mean point $(22.8, 11.5)$ and draw a straight line through it that follows the trend, with about as many points above it as below. A good line passes near $(14, 19.2)$ and $(33, 2.6)$ .
(d)
From your graph in (c), determine the: (i) relationship between $x$ and $y$ ; (ii) value of $y$ when $x$ is 24.

Worked solution (try it first)

Mark $(22.8, 11.5)$ and draw a straight line through it with the points spread evenly on either side.