This comes from subtracting the two averages: 84 − 80 = 4. You cannot subtract averages directly. Find the total for each group: original total = 5 × 84 = 420, new total = 6 × 80 = 480, sixth number = 480 − 420 = 60.
PRIMARY 6 · MATHEMATICS
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This uses 5 instead of 6 as the count for the new group: 84 × 5 − 80 × 5 = 420 − 400 = 20. After adding the sixth number there are 6 numbers, so the new total is 80 × 6 = 480.
This uses 6 instead of 5 as the count for the original group: 84 × 6 − 80 × 6 = 504 − 480 = 24. The original group had only 5 numbers, so the correct original total is 84 × 5 = 420.
Assumed each of the 3 new boys must weigh the new average 44 kg.
Used the increase of 4 kg in average multiplied by 9 boys = 36 kg, then divided by 3 = 12 kg, then added to 36 to get 48 kg.
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