The arithmetic mean of a set of numbers is 12. The mean of another set of numbers is 15. If the combined mean of both sets is 12.5, then what will be the ratio of the frequency of both the groups?
- AOption (a)
3 : 1
- BOption (b)
5 : 1
- COption (c)
3 : 2
- DOption (d)
5 : 2
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Correct answer
5 : 1
Why this answer is correct
Shortcut Trick
By using the rule of Alligation:
Ratio of frequencies = (Mean of Set 2 − Combined Mean) : (Combined Mean − Mean of Set 1)
⇒ Ratio = (15 − 12.5) : (12.5 − 12)
⇒ Ratio = 2.5 : 0.5 = 5 : 1
∴ The correct answer is 5 : 1.
Alternate Method
Given:
Mean of 1st set of numbers (x1) = 12
Mean of 2nd set of numbers (x2) = 15
Combined mean (x) = 12.5
Formula Used:
Combined Mean (x) = (n1x1 + n2x2) ÷ (n1 + n2)
Where n1 and n2 are the frequencies of the 1st and 2nd set of numbers respectively.
Combined 12.5
Set 1 Mean
12
Set 2 Mean
15
15 − 12.5
2.5
12.5 − 12
0.5
Ratio = 2.5 : 0.5 = 5 : 1
Calculations:
⇒ 12.5 = (12 × n1 + 15 × n2) ÷ (n1 + n2)
⇒ 12.5n1 + 12.5n2 = 12n1 + 15n2
⇒ 12.5n1 − 12n1 = 15n2 − 12.5n2
⇒ 0.5n1 = 2.5n2
⇒ n1 ÷ n2 = 2.5 ÷ 0.5
⇒ n1 : n2 = 5 : 1
∴ The correct answer is 5 : 1.
Additional Information
Rule of Alligation
It is used to find the ratio in which two ingredients or sets at their respective prices or averages are mixed to produce a mixture at a given combined average. The formula is: Ratio = (Rate of Dearer − Combined Rate) / (Combined Rate − Rate of Cheaper).
Combined Average
When 'n' groups have individual averages A1, A2, ... An and their respective sizes are N1, N2, ... Nn, then their combined average is evaluated as (N1A1 + N2A2 + ... + NnAn) ÷ (N1 + N2 + ... + Nn).






















