Explanation:
Because Mean = Total of ValuesNumber of Values, you can rearrange this formula to find the total of any group whenever you are given its mean:
Total = Mean × Number of Values.
In any reverse mean or combined mean question, your very first step should always be to multiply each given mean by the number of items to find the totals, then add or subtract the totals as required.
Example 1: The mean mass of 5 parcels is 14 kg. When a 6th parcel is added, the mean of all 6 parcels becomes 15.5 kg. Find the mass of the 6th parcel.
Solution:
Total of 5 parcels = 5 × 14 = 70 kg
Total of 6 parcels = 6 × 15.5 = 93 kg
Mass of 6th parcel = 93 − 70 = 23 kg
Answer: 23 kg
Example 2: In a class, 12 boys have a mean score of 65 and 18 girls have a mean score of 70. Work out the mean score for all 30 students.
Solution:
Boys total = 12 × 65 = 780
Girls total = 18 × 70 = 1260
Combined total = 780 + 1260 = 2040
Overall mean = 2040 ÷ 30 = 68 → Answer: 68