| Mid value |
Frequency |
Class average x
frequency |
10
30
50
70
90 |
17
a
32
b
19 |
170
30a
1600
70b
1710 |
| total |
120 |
3480+30a+70b |
mean = 50
\(\therefore\) \(\cfrac{3480+30a+70b}{120}\) = 50
30a + 70b = 6000 – 3480
30a + 70b = 2520 (1)
17 + 32 + 19 + a + b = 120
68 + a + b = 120,
a + b = 52 (2)
(2) x 30 => 30a + 30b = 1560
30a + 70b = 2520,
30a + 30b = 1560,
40b = 960,
b = 960/40 = 24,
a + 24 = 52
a = 52 – 24 = 28,
\(\therefore\) a = 28, b = 24
