Q.Given that the combined (weighted) mean of the following frequency distribution is 50 and the total frequency is 120, find the values of \( f_1 \) and \( f_2 \). | Class Interval | 0–20 | 20–40 | 40–60 | 60–80 | 80–100 | |----------------|------|--------|--------|--------|---------| | Frequency | 17 | \( f_1 \) | 32 | \( f_2 \) | 19 |

Class IntervalMidpoint \((x_i)\)Frequency \((f_i)\)\((x_i f_i)\)
0–201017170
20–4030\(f_1\)\(30f_1\)
40–6050321600
60–8070\(f_2\)\(70f_2\)
80–10090191710
Total \(\sum f_i = 68 + f_1 + f_2\)\(\sum x_i f_i = 3480 + 30f_1 + 70f_2\)
Given: \[ 68 + f_1 + f_2 = 120 \Rightarrow f_1 + f_2 = 52 \quad \text{(i)} \] Also, \[ \frac{3480 + 30f_1 + 70f_2}{120} = 50 \Rightarrow 3480 + 30f_1 + 70f_2 = 6000 \Rightarrow 30f_1 + 70f_2 = 2520 \Rightarrow 3f_1 + 7f_2 = 252 \quad \text{(ii)} \] Now subtracting \( 3 \times \text{(i)} \) from (ii): \[ 3f_1 + 7f_2 - 3f_1 - 3f_2 = 252 - 156 \Rightarrow 4f_2 = 96 \Rightarrow f_2 = 24 \] Substituting into (i): \[ f_1 = 52 - 24 = 28 \] ✅ Therefore, the required solution is: \(f_1 = 28\) and \(f_2 = 24\).
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